It is entirely a different issue to omit a Mustahab on Shari grounds. For example: Omitting the Mustahab in order to indicate to others that the act is not Wajib (compulsory); in a journey to omit the performance of Nafl Salaat in consideration of ones travelling companions; or because of any difficulty occurring in some essential work; or due to much tiredness, the Mustahabs are omitted. Such omission of Mustahabat is not reprimandable. Even the hadith says:
Verily, your soul too has a right over you.
But the hadith orders us to seek protection against abstention from Mustahabat due to indolence.
O Allah! Verily, I seek refuge with You from weakness and indolence. (Hadith)
It should be well remembered that Rasulullah (sallallahu alayhi wasallam) exhorted us to seek comfort and in its pursuit, he (sallallahu alayhi wasallam) exhorted certain Sahaaba to abstain from even some Mustahabat and reduce the performance of Nafl Salaat. On the contrary he exhorted the seeking of protection from indolence. There is a difference in these two occasions of abstention from Mustahabat. The quest for comfort is after one has made full effort and laboured fully in accordance with ones capability, When one has done so, the Shari command is that one should not labour and toil more than could be borne by ones ability and strength. The order is then to take rest and comfort. If, on the other hand, one labours a bit and not to ones full capacity, and then refrains from the work, then it will be said that the cause is indolence. It is of such indolence that Rasulullah (sallallahu alayhi wasallam) ordered the seeking of protection.
Since our relationship with Allah. Taala is solemn, great and vital, the execution of Mustahabat too is vital. This discussion was initiated by the assertion that every part of the Quran at Allah Taala is of importance. It was said in this regard that in the Quran Kareem is the mention of Mustahabat, but inspite of this, these Mustahabat are considered to be unimportant; it is necessary to educate and instruct in these; their benefits and significance are innumerable. One such benefit is that at times Mustahabat act as a prevention against sin. One who is constant in Tahajjud, Ishraq, Chasht and Aw-wabeen will abstain from sin to a greater degree than one who performs only the five Fardh Salaat. Furthermore, a regular performer of Tahajjud, etc. is considered a pious person by others. This consideration in itself acts as an impediment to sin since such a person ,will feel ashamed to perpetrate sins.
Another benefit of Mustahabat is that sometimes Allah Taala is pleased so much with a certain Mustahab act that najaat (salvation) is obtained by virtue of it.
An interesting episode is related here to indicate this importance. Someone saw in a dream, Seebaway, the expert of Arabic grammar, and enquired of him as to how he fared by Allah. Seebaway by belief belonged to the Mutazili sect. he replied: Allah forgave me. When asked: On what basis did Allah forgive you? he replied: On the basis of a question in grammar. He outlined the question as follows:
The experts of grammar have differed in regard to the question of Marifah (proper noun). Among the Dhama-ir (pronouns which are a class of Marifah) which Dhameer is Araful Ma-arif? Some said that Ariful Maarif is the Dhemeer of the Mutekellim (1st person the speaker); others said the Mukhktab (second person the one who is being addressed.) And I said that the word Allah is Ariful Maarif; that there is no Marifah superior to it, because the word Allah refers to the Being of Allah exclusively, there being no other possibility. This was pleasing to Allah Taala Who said:
You have honoured MY NAME well. Go you are forgiven.
Thus it will be realized that the forgiveness and salvation of this grammarian were attained on something which he said without even having had the intention of gaining reward, but he obtained victory on its basis.

Episode NO. 2
A saint was walking once at night time in severe cold. In the dark he saw a kitten shivering in the cold. He took pity on the kitten and brought it home where it was warmed. After the saint died, Allah Taala asked him:
What have you brought for me?
He ruminated: My deeds are not worthy of presentation, but by grace of Allah, I possess the treasure of Iman in which there could not have been any trait of ostentation (riyal). I should present this to Allah Taala.
He then stated: I have brought Tauheed.
He was then reminded: Do you remember the night of the milk? The night when you drank milk and attributed the ensuing stomach-pain to the milk, saying that: the milk caused the pain. Whet! Is this Tauheed? You overlooked Me and attributed the act to the milk whereas I am the true Cause.
Upon this revelation, the saint trembled in fear. Allah Taala exclaimed: You have now realized the reality of your claim? Now, l will forgive you because of an act which you never dreamt would be the medium of salvation. One night you took pity on a kitten shivering in the cold end gave it succour. You were merciful to My creature. It is a greater requirement that I be merciful to you. Go, you have been forgiven.
Such is the grace, barakaat and favour of Mustahabat. The ahadith are replete with such instances such insignificant occurrences which countenanced salvation.
Another illustration is the episode of a prostitute, which has been narrated in the hadith. Once she saw a dog in the intense heat licking the ground due to thirst. She took pity on the dog. Nearby was a well, but there was no rope and bucket with which to draw the water. She used her scarf as a rope and her leather sock as a container. With these she managed to draw sufficient water to quench the thirst of the almost dying dog. After a while this woman died. Rasulullah (sallallahu alayhi wasallam) said that she was forgiven and attained salvation because of this merciful act shown to the dog, It has been well said:
The Rahmat of Allah is in search of some excuse so as to forgive (people their sins). It does not search for any price.
My friends! Never regard any good act as insignificant or despicable. It is not known which act appeals to Allah Taala. Nowadays there are people who are disposed to abstain from practicing righteous deeds upon hearing episodes of Allahs boundless mercy.

The similitude of these episodes is like the rain. Not only these episodes, but even Nusoos (Quranic verses and Ahadith) bear the same similitude. Rain in itself is highly beneficial and life-giving. However, its effect on different substrata varies. If the ground is fertile, the beneficial effect is manifested in the luxurious growth. If the ground is arid and barren, then the more the rainfall, the more thorn-trees, brambles and shrubbery. Shaikh Sadi (rahmatullah alayhi) says: there is no doubt in the beneficial effect of rain. If it rains in a garden, tulips grow; if it rains in arid ground, thorns and brambles glow.
Similar is the effect of episodes of Rahmat on different persons. People of different dispositions are affected differently by these stories of mercy. Those of a sick disposition and unhealthy temperament interpret these incidents of mercy in a grossly distorted manner. They say if Allah Taala forgives on the basis of such trivial acts, then there is no real need for righteous deeds. On the other hand, those of a healthy disposition and who are straight-thinking, upon hearing such incidents of Rahmat increase their acts of obedience. In fact, at times, if forgiveness is obtained without punishment, those whose hearts are imbued with Divine Recognition are smitten with shame far greater than the shame which they would have suffered if they had received some punishment. Only those who have suffered this state know its reality.

Whoever has understood this condition (of extreme shame for obtaining forgiveness without being punished) will understand without the slightest difficulty the tafsir of the ayat:
Thus, (Allah Taala) awarded you with grief as a recompense for grief so that you may not (again) grieve.
The reference in this verse is to an incident, which occurred during the battle of Uhud. Prior to the commencing of the battle, Rasulullah (sallallahu alayhi wasallam) posted fifty Sahaaba to guard the mountain pass to thwart any attack from the rear. They were instructed not to leave their posts without Rasulullahs (sallallahu alayhi wasallam) permission, no matter what the condition of the main army may be. After this arrangement Rasulullah (sallallahu alayhi wasallam) ordered the Muslims to attack. By the fadhl of Allah the Muslims gained victory within a very short while. The kuffaar were decisively routed. Abu Sufyan Ibn Harb who at that time was the commander of the kuffaar army, fled with his army. The flag, which he was holding, fell down. The Muslims pursued the fleeing enemy. On observing this total route of the kuffaar, a difference arose among the Muslims guarding the mountain-pass. Some were of the opinion that since victory has been achieved there no longer remained the need to guard the pass, as the purpose for this duty no longer existed. In their opinion they would not be disobeying the order of Rasulullah (sallallahu alayhi wasallam) if they now left ┬┐their position. They argued that thus far they had not actively participated in the battle and they now felt the need to join in the pursuit of the kuffaar. However, some others opposed this view, saying that Rasulullah (sallallahu alayhi wasallam) issued strict instructions not to leave the post under any circumstances and without his express consent. But, the first group rejected this advice and forty men left the position, which they were guarding, and became engaged in the gathering of booty. This was an error of judgement on their part.
Khalid Bin Walid who at that stage had not yet embraced Islam and who was in the army of the kuffaar observed this move. He had left some spies at the mountain to keep him informed of the position at the mountain-pass.
While he was on the run with the army of the kuffaar his spies informed him that the fortification at the mountain was abandoned. Khalid Bin Walid who was well-experienced in military tactics and a seasoned officer immediately retrenched his steps with five hundred soldiers and soon reached the post now guarded by only eleven Sahaaba. The Sahaaba defended valiantly but were overwhelmed and martyred. Khalid Bin Waleed then commenced his attack on the Muslims from the rear. Seeing the tables being turned, the rest of the kuffaar army joined in the attack on the Muslims who were now encircled. The danger which caused Rasulullah (sallallahu alayhi wasallam) to fortify the mountain-pass now materialised. Seventy Muslims were martyred in the ensuing engagement.
Hearing the false cry of the shaitaan that Muhammad is slain, the feet of many Sahaaba were uprooted and the tables were completely turned on the Muslims. Inspite of this severe setback the Muslims were not defeated. Sayyiduna Muhammadur Rasulullah (sallallahu alayhi wasallam) with a handful of devotees held their ground, never turning their backs. Rasulullah (sallallahu alayhi wasallam) ordered the Sahaabi to regroup. With one call all the Muslims regrouped.
Allah Taala attributes the calamity upon the Muslims to the error in judgement, which they made when they abandoned their position without the permission of Rasulullah (sallallahu alayhi wasallam). The Quraan, therefore says:
And, you disobeyed (the order of Rasulullah) after He (Allah) showed you that which you loved (i.e. the victory of the Muslims.
Allah Taala then reprimands them:
Thus He awarded you with grief as a recompense for the grief (which you caused Rasulullah by departing from his command) so that you may not grieve over that which has slipped by you.
The wisdom underlying this retribution is stated:
So that you do not grieve over that which has slipped by you.
This digression was necessitated by the assertion that some persons of noble disposition undergo greater shame and regret if no retribution is taken, hence the purport of the ayat (mentioned above) is:
We recompensed you with a little hardship so that you may not grieve much, if forgiven without retribution having being taken.
The Sahaaba were the beloved devotees of Allah and Rasulullah (sallallahu alayhi wasallam). If their error was forgiven without any recompense being meted out, they would not have been able to raise their heads all their life because of regret and shame. Thus they were given a slight punishment so as to save them from great grief. It will now be realised that punishment is not always to inflict grief. On the contrary, it is meted out sometimes to reduce grief.