The side impact protection works pretty well (speaking from experience ) I'd often wondered about it too, but you would have to be in an unusual crash to hurt your jaw or cheekbone.

The little bit of side protection helmets provide is more than adequate for your everyday, garden variety bike crash where your shoulder hits the road first, followed by your noggin. Your shoulder keeps the lower part of your head away from danger so your delicate features are preserved but in doing that it catapults your brain cage into the ground, which is where the helmet comes in.

I have only ever seen BMX riders hurt their face in a crash. Takes a certain breed to pull of something that strange and difficult.