John brings up a very valid point. There is no argument that rear disc brakes look good through an open wheel and they perform better under severe braking conditions. So except for the looks the average street driven vehicle has nothing to gain from rear discs. A call to Master Power Brake in North Carolina will confirm this. They sell rear disc kits for almost anything and they will tell you straight out that unless the vehicle is being road raced a good set of drum brakes in the rear is all you'll ever need on the street.
Weight of the components and simpler repair and replacement is the biggest reason you find them on all 4 corners of most production cars today. It certainly isn't because your new Oriental Econo Box is a hard core canyon carver.
Mike