Nothing is disabled, and you don't lose any functions. You can still input flight plan waypoints, etc on the PFD. The only thing you "lose" per se is the big moving map and the schematic flight plan page, but they are available with the push of the reversionary mode button if you need a quick look. Nothing wrong with switching back and forth either. I instruct on G1000-equipped C172s, and we switch back and forth all the time.