One thing that I have noticed over the years is that a lot of people assume that brakes are brakes. The same people who would not pick a random set of heads or camshaft for their engine will bolt random brakes on their car. The brakes need to be selected with as much care as the camshaft. The caliper bore sizes need to match the weight distribution of the car and the master cylinder bore size needs to match the caliper bore size. It is a hydraulic system and all of the piston sizes need to be carefully selected for correct operation. Most of the people I talk to have no idea what the piston size is in their calipers or the master cylinder. They just bought some cheap parts off the internet and bolted them on.
My takeaway then, if they ain't complaining, they probably didn't need rear discs performance wise in the first place, so no real harm evidently.
Rear discs even on the rear is seldom needed. For example, when is the last time in street use do you ever remember even smelling your drums from repeated hard brake use?
4 wheel disc IMO found OEM favor mainly in their better application in ABS systems, the weight decrease was a side benefit.
