I agree with most of what's been said in this thread, even the line about a helmet being a good place to attach a mirror, but it has nothing to do with looking "cool" or anything, just that it's a good way to raise the probabilities of surviving a bad accident.

I had one about 20 years ago. The helmet transferred the forces down to my clavicle, which fractured. That healed in a few weeks, but the alternative would have been my skull, which would have been fatal or nearly so. I naturally owe my life to that cheap piece of plastic and styrofoam. It indeed was cheap compared to the job it did.

There was a reference to the Tour de France and why some of the bikers don't wear their helmets all the time. Well, it puzzles me, too, especially after seeing the piece here:
http://www.sph.emory.edu/Helmets/boardman.html
Surely every participant of the Tour is intimately familiar with that bit of history.

Needless to say, I feel naked if I get on my bike without my helmet. Perhaps it's the mirror that makes it feel odd without it. It doesn't bother me one bit that I've been wearing the helmet for so many years and I've only used it once! So far!