Probably a lot less relevant for MTT players than for cash players. Extremely relevant for STT grinders for which rakeback is often the largest part of their winnings. Aguably, rakeback is more important for STT than for NL cash. And in cash games, rakeback is less and less important the deeper you play (crucial for shortstackers, not so much for deep table regs)
Essentially, the games where you can put in a ton of volume are also games where you have small edges, so rakeback is more important. The games where you can have a massive edge won't allow you to put in a lot of volume, so rakeback matters less.