I find most of my FO's hand fly to about 18,000 feet. When asked, they don't have a particular reason why FL180 is some sort of magic altitude, they just seem to think that is an appropriate altitude to, "turn the motion off."

I would argue that down low is the most important time to be on AP. I generally put it on around 1000' and spend more time looking outside. I can fly the AP (737) very smoothly, more smoothly than I can hand fly. Sure, if I really try, I can hand fly smoothly but it's too much effort. Good AP technique takes practice and I believe the AP can be smoother the vast majority of pilots.