I have a vintage Bernina 830 and never used the knee lift until last year when I started FMQ. Now I use it for all of my sewing. Tonight I'm working on a Sewing Workshop blouse pattern that has several right angles in the front yoke and collar, which have to be reinforced and faced. It's so much easier to use the knee lift at each pivot.
I'm only comfortable with using my right foot and right knee for the foot pedal and knee lift. I found that the foot pedal needs to be really stable so it stays put once you find the best spot for your foot. If it scoots around, I'll bump the knee lift when I don't mean to.