I, personally, wear a helmet whenever I ride. The mental association is such that if I drive or walk down certain streets, I feel like there ought to be something on my head. But I also make sure that people know that helmets are not compulsory for adults in California.
In the end, life is generally hazardous. By rights, we should have a uniquely designed helmet for every exercise we do, including a showering-helmet that lets us wash our hair in the shower while being protected in case we slip, a stair-climbing-helmet, and others. And, as far as bike helmets, we're talking about percentages of percentages. You have a X% chance of getting into an accident. If you do, you have a Y% chance of benefiting from a helmet. I'm using variables, because there's very little actual consensus on the numbers.
Thus, all things considered, improving X is much more of a benefit than improving Y. This is not as clear-cut as you think. For example, more cyclists on the road improves the safety for cyclists. If a driver has a near miss or is accustomed to seeing bikes in traffic, they'll be more careful. With enough bike commuters and utility cyclists, eventually this even creates a reduction in the number of cars on the road.
If people are used to not wearing helmets for an activity, it is viewed as safe. Thus, we think that it's safer to walk a mile instead of bike a mile, even though it's actually the other way around. Likewise, the lifespan improvement from biking instead of driving for your average cyclist is awfully good even without helmet usage.
A widely quoted study shows that in Australia, helmet laws improved cycling safety... largely because people just stopped cycling altogether. So as far as I'm concerned, if being freed of the worry of what a helmet will do to a person's hairdo on the way to work will increase the number of people who include biking into their commute, they get a free pass from the "where's your helmet?" question. They'll still be healthier, happier, and perhaps even safer than if they were driving a car.
There's some freaky side effects. There's a study where they found that cars drove closer to cyclists if the cyclist was wearing a helmet.
In the end, cars and motorcycles can reach speeds that your average recreational, utility, or commuter cyclist won't reach so helmets and seatbelts make perfect sense. But cycling is a different beast entirely. Likewise, Y% is not as big as you'd think. If you wear a helmet built more like a motorcycle helmet, you will get much improved head protection, but you also will be quite uncomfortable.
Still, I think it's a very good thing if people do wear bike helmets. There's tons and tons of broken helmet pictures on the Internet. Especially in terms of things like road racing and mountain biking, where you really do run the risk of getting into an accident.
The part that's irresponsible is that there are other laws that can be passed and awareness programs created to make cycling safer that will have much more positive of an effect. So, yes, helmets are good. But so is having lights so you can be seen at night, riding on the correct side of the street and not the wrong way on the sidewalk, not having your iPod on, requiring cars to maintain a decent clearance from cyclists, and building bicycle bypasses.