I always use the MGs on the highest RPM. I assume most people don't because the recoil can be hard to control when under duress, and you can get into a loop of freaking out, losing control and missing all of your shots, dying, and saying "fuck it" as you lower the RPM. I've been there, but when messing around I found out how truly satisfying it is to mow down entire crowds with any of the MGs on the highest RPM.
Plus, if Sweet Lady Liberty didn't want me to send Freedom inducing lead at 750RPM, 900RPM, or the fabled 1150RPM, then the armourer wouldn't have put that option on the gun