Compositing Layers are usefull for more than just compositing. For example I have created a Flash file where I loaded all rendered composite layers seperate and through a mouseclick the object(s) in that layer dissapear. And besides having a perfect fit, the complete result is rather small (44KB, 166KB worth of PNG files).
Is it called DeCompositing?
http://albartus.com/blender/composite1.htm You can click different items and they will dissapear This is just a test to see how usefull it can be.