What is the ruling on one who abandons the emphasized Sunnahs, and is he sinful?

Confirmed Sunan (Prophetic traditions) do not incur sin for the one who neglects them, but being lax concerning them can lead to laxity in fulfilling wajibāt (obligatory duties). They protect the farā’iḍ (prescribed duties) and compensate for their deficiencies. Thus, whoever preserves them keeps away from neglecting obligatory duties. They compensate for what is lacking in the farā’iḍ, as stated in the hadith: "The first thing a servant will be held accountable for is prayer..." And in it: "The Lord, the Mighty and Majestic, will say: 'Look, does My servant have any voluntary prayers (tatawwu')?'" Therefore, one should not be lax concerning them.