Maybe not the best reason - or beat a dead horse, but I have mounted the rear brakes in the forward position when the brake reach was not quite there in the other position (i.e. short reach brakes that did not quite make it as rear facing). Usually this was while I waiting on a drop bolt. Depending on the frame, sometimes this worked. I am partial to suntour superbe - but that is just me.
I have heard that short reach brakes are optimal to long reach. If they are off by a smidge (technically speaking of course), a change from rear facing to front facing sometimes gets you there. The honest reason for me is that I've purchased the wrong brake, but made it work... Just another perspective.