It allows you to select split vertex separately, like selecting independent vertex normals, or UVs in the 3D view, can also be used to apply different vertex colours to one vertex (but there are usually easier ways to go about this)... I never use it. :)
Am I missing out on some sort of functionality by not using vertex face at all in Maya? I don't see what the purpose of blowing out every face is. I would say it's easier to see different faces but, in my opinion, it just makes it worse. I've just been looking at it on the flick menu's for the past few months every time I…
I have used it for things like vertex lighting/coloring. http://blog.environmentartist.com/?p=205 Warheart is right. It's good for pointing things out if you don't want to do a cleanup.
It also lets you check if you have accidentally created faces with no surface area (e.g. extruding and not applying any offset) they show up as a line in vertex face mode. Although you can fix this with the cleanup tools or merge verts without having to actually see it. So another semi-useful use :P.