Quote - 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 need to sit down one day and really try to wrap my head around Alpha Channels and Mask Renders.  They both seem to be used by many.. I just dont quite understand them right now.