Well, I've ridden thoroughbreds since I was 3yo, so I have some experience with being thrown through the air, and the concussive results of that. I've also crashed my bike, with and without the involvement of other vehicles. In my worst crash, I hit a concrete bollard head-on while pedaling at speed. I flipped over the handlebars and landed on my back. My tucked, helmetless head never touched a thing. I destroyed the heavy-duty suspension fork on my hardtail, but other than that I just scraped my elbow and got my office shirt dirty.

Another occasion that really sticks out in my mind is the time I laid the bike down and jumped off to the side to avoid a child who literally jumped out from behind cover into the street directly in my path. If I had been in a car, going the same speed, that episode would have ended very differently.

I don't have a problem with helmets in general. I never snowboard without a helmet. There are many rivers I won't kayak without a helmet. I don't usually wear a helmet when riding horses, but there are exceptions, like polo. I'm sure I'll encounter other circumstances that seem helmet-appropriate as well. The point is, bicycle commuting is not an extreme sport, so enforcing the conventions of extreme sport is an effective way to discourage it.

For every anecdote, there is another anecdote. You only think your helmet saved you, while I know that my lack of helmet did not harm me in the crash I described. (And if we're going to speculate intuitively, it's a good thing I didn't have a solid bulky object on my head to catch on the pavement and impart a jolt of rotational acceleration to my brain!) If you've never considered the phenomenon of risk compensation, you could think more deeply about an issue that is much more complicated than "it's bad to injure your head". Fortunately, there's a place you can go to educate yourself: http://www.cyclehelmets.org/