Just read an interesting article that brings up the question of whether bicycle helmets actually hurt or help the movement to get more people on bikes. http://www.nytimes.com/2012/09/30/sunday-review/to-encourage-biking-cities-forget-about-helmets.html
My take on it is that you should wear one if at all possible. Bicycle helmets are not overly costly for an entry level model that will protect you should you fall. I do agree, that we should probably lose the stigma of looking down on those who are not wearing helmets. I think you need to weigh in the type of riding that someone is doing. If your just cruising the streets for fun or if your going for speed. If your decked out in spandex, I expect you to be wearing a helmet, but if your casually pedaling along, I wouldn’t hold it against you to be going helmetless. The other great point of the article is that cities where they have bicycle helmet laws are very far behind in cycle adoption. To have a thriving cycle community you really need to be open to not forcing the helmet issue. Alot of cities have bicycle rental stations where you can just rent a bike from an automated rack. This kind of rental would never work if you were to force helmet laws on the renters.
In the end I think that bicycle helmet use should be a personal choice.