I used to use multiple RGB color separation passes as mask for tweaking some still image in post (Nuke), and started feeling a bit cumbersome when I need to go back in Maya off-and-on to render specific RGB mask or use "Bezier" to create custom mask based on "currently-have" RGB passes. Since I am going to deal with animated objects later on, it is going to be painstaking process.
However, I heard that "label ID" pass is often lack of color accuracy and hence cause artifacts. I have seen some aliasing issue discussed in different forums.
I am wondering, in real production, will you use the RGB mask or label/material ID pass as mask in compositing especially when you are also dealing with animation which has motion blur? (I plan to generate motion vector pass and add in MB in post)