Hi Adam,
I was afraid of s.o. asking this!
Some reasons against PS
1. Photoshop has issues with the correct compositing of render passes with alpha channels. There are black seams around parts of different material or pass information. Even wth premultiplied Alpha OFF(;-))
2. I cannot use CC, like I know to do with AE, Toxik or formerly in Combustion
3. I cannot use keylight to key the color matte passes for correcting some object parts separatly. Or is there a keylight for PS
But how do people work in 4k when compositing 3d animation in AE?