Piston motors generally have too much compression for a prop to freewheel, although I have a K&B 40 that will.
Electric planes need some way to halt the prop, as a turning prop adds drag by turning the motor.
The motor can act as a generator, but the amount of power it puts out is too small to be of any use.
Getting rid of the drag of the turning prop is better.
Either a folding prop, or electric braking.