The idea of a winch on the front, at least on a jeep, is to self-extricate. And, at least on jeeps, there is a nice little mounting area right on top of the front bumper. In the rear, it may get in the way of the tailgate. Also, you gotta run it from the battery (or a separate battery): easier to only run a little wire, through the front grill, into the engine compartment, instead of through the whole damned vehicle.
Other than that, I've never really given it a second thought.