Its more a safty measure than a feature but if it could be done that would be nice:
Imagine you render an animation. After its finished you find out a mask wasn't correctly setup,
so you quickly fix that in the scene. You are in a hurry and just check the render only masks checkbox and hit render.
5 minutes later you realize what you did but its too late and 100 frames of renderelemetns that require shading are overwritten with black images.
So if those renderelements would automatically be disabled when checking the "render only masks" feature or just would not be written even if activated
this could save some trouble.