Consider a bike with horizontal drop-outs. In such a case, the brake mounted conventionally (rearward of the seat stays) is likely to tolerate more back-and-forth adjustment of the rear wheel w/o needing brake pad adjustment than one with the brake mounted forward. Of course, with vertical drop-outs, it doesn't matter.
The only time I was tempted to mount forward of the stays was, as I recall, late one night with a caliper too long for the conventional position. Otherwise, I've left well enough alone.
your mileage may vary.