In my case, it's just a question of expense - a good, heavy duty grinder with a motor can be a good chunk of change. Taking the motor out of the equation can cut down on the cost quite a bit, which makes sense to me if I'm going to be using it only occasionally.

To answer your second question - yes, grinders can heat up beans to the point that it's detrimental (if you're pretty picky about these things). At a guess, I'd think that this is more a function of dull grinders pulverizing as opposed to cutting, rather than a question of heat generated by the motor.