Hi,
I use it mostly in product rendering animations where some parts needs to be changed, but the rest is the same.
Like a wallpaper changing in a room, first you make a render from the whole room, then apply a RaySwitch material with black mtl in the directoverride slot
to everything exept the walls. Now you change the wallpaper color or material. The room will be black but the GI, reflectons, etc. still calculated from the
original materials. Than it can be composited together. This way I can reduce the rendertimes significantly in heavy scenes.
Unfortunately Multi sub not supprted at the moment, so it can be tedious when you have to setup a lot of materials.
The object level version of this (matte/shadow properties, like in Vray) would be very handy in final V1.
I hope this makes sense.
Edit: corrected attachment according to Rawalanche