I'm of the dame mid as Dries, as I do this quite a bit when making small changes to larger renders. It saves a ton of time, and the fact that I can drop in a region render file into a current PSD that I've already put a ton of work into and not have to line it up manually is quite handy.
I guess I could see making it optional, and would be part of the post processing part of the render process to have the image clipped to non-transparent pixel boundaries, but I don't really see the benefit.
I did not know that about the exr format, thanks to Soren for that bit of info. I've been playing with exr outputs and trying to see if they would provide any improvements over my standard 8bit psd files, but I'm not sure I'm there yet. No heavy compositing, just simple post work.