Luckily there is no helmet law here....so I wear it only if I'm gonna go out of state. My wife on the other hand won't ride without one, but she also rides her own bike so it doesn't bother me. I would never ask her not to wear one beacause then I would feel reaponsible if something happened to her.
I guess I just like a certain level of risk in my life and I'm comfortable with it.
If you want to read some interesting studies on the whole helmet issue, you can look here:
Until recently, my state (SC) was the only one in the Southeast that didn't require one. Of course, not too long ago, FL repealed their helmet law. Now, when I ride to Daytona, I just try to sneak through GA!
I never wear one unless I'm in a state that requires it. And the little pea-pod I have that passes for a helmet wouldn't do a damn thing anyway!
I'm a firm believer in "Let those who ride decide" I NEVER drive without a seatbelt though.