CC composite is great for compositing its own layer over itself again, the track mattes is good for tracking an alpha/luma of a layer above itself.
However, many times I'd like to composite, say an icon, over a circle shape - then animate each one independently (perhaps the circle scales up and the icon slides down to the centre of the circle), then, have the icon track the circles alpha channel so you only ever see the icon inside the circle.
Set matte doesnt often work as you are manipulating transform properties, and set matte seems to only work when both layers are the same dimensions or their default transform properties otherwise the matte is out of alignment.

The track matte works but you have to duplicate the circle layer and put it above the icon, then alterations to the circle animation would have to be replicated (barring fiddly pick whipping).

So, the feature would be like a track matte for any layer, essentially (could remove the track matte switch).
I think it could be a setting or check box in the "Set matte" effect, simply "track matte" which would track any layers matte/luma etc, as it appears in the composition, like it has been pre-composed but without having to pre-compose.

I hope this makes sense.