How often do you use f/2.8 on your FF cam?

Hello FF users, I have a question for you guys.

How often do you use f/2.8 or faster on your FF cameras?

I have been using m4/3 for a while and am considering getting a bigger sensor camera for those interior/low light shots with tiny DOF that are impossible on a 4/3 sensor. I am considering either getting a APS-C camera, OR a used Canon 5D Mark I ( 800$) with the Tamron 28-75mm f/2.8.

I would like to know if the f/2.8 in the 28-75mm range, or even f/1.8 on a 50mm prime are that often used? Are these apertures that useful?

I just wanted to make sure I won't end up stopping down to f/3.5+ most of the time to keep whole faces in focus, TOO blurry backgrounds, etc... In that case I might as well just get a fast lens on APS-C for a similar price and lighter weight.

Any comments? Can any die-hard FF fans explain to me what are all the great and irreplaceable advantages of FF and convert me over?

Thanks!