I wear one because it's required to ride on the installation I work on, the club I ride in, and the races I ride in.

Plus, I'm a father, and if it increases my odds of getting back to my family by even a fraction of a percent when some bad **** happens out on the road, it's worth it. Most of the reasons that call into question helmet use (makes cycling seem dangerous, distracts people, makes you more likely to crash, etc) apply to people in general (society) but not individuals who are experienced riders.

That being said, I'm also a liberatarian, so make your own damn decision. But don't be one of those people who ride around with the helment on thier handlebars instead of thier head - that's just dumb.