I always wear a helmet. All other things being equal, wearing a helmet is a lot safer than not wearing one. I will probably continue to wear a helmet, particularly here in the US. However, I hate them; they are hot, uncomfortable, and look stupid. When you do the math, their protective value, while very real, may, for some experienced urban riders, not be worth the discomfort. This may be particularly true for those of us who use bikes as our primary form of transport.
In a serious accident on a bicycle, the value of a helmet is simply beyond question. Go to any serious website, ones where they give actual data, and you'll see that having a helmet in an accident is a really good idea. The thing is, those serious accidents on a bike are very, very rare. According to the League of American Cyclists, there were 4 billion bicycle trips in the US in 2010. During that same year, according to the CDC, about 500,000 people sought medical attention because of bike accidents, and about 780 people were killed. Even assuming that all those ER visits involved serious head injuries, the probability that any one of us will be involved in a serious accident on a bike in 20 years of riding is about 0.25%. Even if you account for a much higher number of trips made by car-free cyclists, at most, any one of us has maybe a 1 in 100 chance of being in a serious accident over a span of 20 years. If you accept the figure that a bicycle helmet has an 88% probability of protecting your head in an accident, that means that, over a span of 20 years, wearing a helmet increases your margin of safety by just under 1%. (0.88%)
As I said, I'm going to continue to wear a helmet; it's safer than not using one, and even though it sucks, it sucks less than permanent brain injury. But it's certainly no guarantee of safety, and an experienced, careful rider may opt for 1% more risk in exchange for greater comfort. What do you think? Are helmets necessary for experienced car-free riders?