Plus on final approach the student needs to be focussing on more important things than carb ht.
Indeed! With the same logic behind putting the carb heat back to cold at 200ft because of a possible go-around one could also retract flaps and gear at 200ft. Makes the go-around much more easy, believe me!
On the C152 we were taught (and still teach) to leave the carb heat on until after touch down and in case of a touch-and-go reset it with the thumb while the other fingers move the throttle forward. Unfortunatley, in a Piper this trick doesn't work. Therefore it goes like this (in our FTO): 1. Raise flaps, 2. retrim, 3. carb heat to cold, 4. apply power. If the runway is too short for that, we don't do touch-and-goes there.
Happy landings,
max