Question: If one performs a sunnat salat with the intention of both performing a sunnat salat and performing a qada salat, one is considered to have performed the sunnat salat as well. One is not considered to have omitted sunnat salats. If one never performs sunnat salats, will one not incur any sin?
ANSWER
I will cite passages from fiqh books about this matter:

1. Ibn Humam stated: "Not performing sunnat salats causes one to miss out on its rewards and high ranks." (Jawhara, Durr-ul-Muntaka, Radd-ul-Mukhtar, Tahtawi)

2. "It is not sinful to omit muakkad and non-muakkad sunnat salats. One who omits them will miss out on only its rewards and high ranks and will be scolded for them" (Halabi al-Saghir).

3. "In spite of the fact that the sunnat salat of Salat al-Fajr is highly emphasized, no punishment was reported for a person omitting it" (Tahawi).

4. "One who skips sunnat salats without an excuse will not be sinful, but one will be questioned and scolded for them" (Se'adet-i Ebediyye).

It is stated in a hadith-i sharif:

"A person with unkempt hair came and asked the Messenger of Allah:
— O the Messenger of Allah, what is Islam?
— It is performing salat five times a day.
— Isn't it more than five times?
— No, but anyone who wants to perform voluntary salat can perform it. In addition to it, you have to fast for a month in Ramadan every year.
— Do I have to fast other than it?
— Whoever wants to fast voluntarily can fast. Besides, a wealthy person must pay zakat for his wealth.
— Is there anything more that I have to pay?
— If anyone wants to give voluntary charity, he/she can give it.
— By Allah, I will do no more than or no less than these.
— A Believer who does these will attain salvation." [Bukhari, Muslim, Abu Dawud, Nasai]

Some scholars who took the last hadith-i sharif as evidence stated that it was necessary to do fard acts of worship completely even if one did not do voluntary acts of worship or perform voluntary salats. Nevertheless, one should not skip sunnat salats of the five fard salats because sunnat salats compensate for the deficiencies in one's fard salats. If one has fard salats omitted, one must perform sunnat or nafila salats with the intention of both performing sunnat or nafila salats and performing qada salats. Some hadith-i sharifs about the importance of fard salats are as follows:

(The best jihad is to do fard [obligatory] acts of worship.) [Imam-i Ahmad]

(While everybody is busy doing voluntary acts of worship, strive to do fard ones.) [Miftah-un-Najat]

(One cannot approach Allah through anything as close as one approaches Him through fard acts of worship.) [Bayhaqi]

(The voluntary acts of worship of a person who has salats to make up will not be accepted.) [Durrat-ul-Fakhira]

Hadrat Abu Bakr stated:
"Allahu ta'ala does not accept the voluntary acts of worship of one who has salats to make up, unless one makes them up" (Kitab-ul-Haraj).

Hadrat Imam-i Rabbani stated:
"The value of voluntary acts of worship compared with obligatory ones is not even like that of a drop of water compared with an ocean" (Vol. 1, 260th Letter).

"If a voluntary act of worship causes one to skip a fard act of worship, then it is not counted as an act of worship, but, in fact, it is harmful" (Vol. 1, 123rd Letter).

Hadrat Sayyid Abd al-Qadir Ghaylani stated:
"The hadith-i sharif 'The voluntary acts of worship of a person who has salats to make up will not be accepted' shows that it is idiocy to spend time performing nafila salats while one has salats to make up. A person performing nafila salats while he/she has debts of omitted salats is like a debtor taking a present to the person to whom he/she owes, which, normally, is not acceptable. A Believer is like a business person. The fard salats are his/her capital, and voluntary salats are his/her profit. Unless the capital is rescued, they will give no profit" (Futuh al-Ghayb).

"Just as it is a major sin to skip a fard act of worship, so it is a major sin to delay making it up. If one does not perform sunnat salats with the intention of both performing sunnat salats and performing qada salats, one is considered to have delayed making up qada salats, which is obligatory, and one is sinful for this. For this reason, one must perform sunnat salats by both intending to perform sunnat salats and to perform qada salats. Thus, one will be considered to have performed both sunnat salats and one's qada salats" (Nawadir-i Fiqhiyya).

Question: What does it mean to say, "Omitting sunnat salats is a minor sin, and insistence on it is a major sin"?
ANSWER
The general rule is this: Insistence on minor sins is a major sin. Omitting a sunnat salat is not sinful. Insistence on omitting sunnat salats without an excuse is a minor sin. One who considers a sunnat salat to be unimportant becomes a disbeliever.

Question: It is written in Namaz Kitabı (The Book of Salat): "It is a sin to never perform sunnat salats." Performing a sunnat salat is not fard or wajib. Then why is it a sin to omit it?
ANSWER
Not performing any sunnat salat without a reason is different from abandoning a sunnat salat out of laziness. Not performing them because one takes them lightly is entirely different from these two. The following is written in books:

"It is not sinful to skip a sunnat salat intentionally. One will not be inflicted torment for this, but one will be scolded. One who abandons sunnat salats will miss out on the rewards given in return for them" (Islam Ahlakı).

"It is not sinful to abandon sunnat salats. However, it is a minor sin to continuously abandon them and to insist on abandoning them. Disliking them or considering them unimportant is kufr [disbelief]" (Diya-ul-Qulub).

It is written in Halabi al-Saghir, "It is not sinful not to perform the sunnat salat of Salat al-Fajr and the other muakkad sunnat salats. One will miss out on only the rewards given for them and high ranks, and one will be scolded for them" (Se'adet-i Ebediyye).