I don't know what kind of image are you working on, but usually I use photoshop to compose in layers a complex image, using in Bryce the "mask render" as alpha channal.
Also in the 3D movies (as Pixar movies and so on) they use layers composition during the postproduction in order to speed up the 3D renders. Sometimes in the movies there are more than 10 layers!

I really think that the usual "mania" of 3D artist on Renderosity,  to render all the 3D scene in one shot is really a MANIA... :) more maniacal if is a still image. ;)
The "layer render" give you more control on colors and effects (and many other things) when you postproduce the layers in Photoshop.