Helmets make sense in certain situations, like road racing or mountain biking, but for casual rides or basic transportation, I think cycling is a pretty safe endeavor which does not necessitate a helmet.
I recently watched a documentary called "Bill Cunningham New York." It is about a fashion photographer for the New York Times who takes candid street photos. His method is to ride a bicycle all over Manhattan and take photos of people wearing outfits he finds interesting. He then publishes his favorites in the Sunday Times. He has been doing this almost every day for something like 40 years. He is now 82 years old, and he is amazingly still at it. During all this time, he has never worn a helmet.
So, his story is a good example of the benefits of cycling. This 82-year-old is fit and healthy enough that he continues to work in a physically active profession. I would say this is due, in no small part, to his regular cycling. Furthermore, the fact that he does not wear a helmet is a pretty good indication that basic cycling is not as dangerous as it is made out to be.
Anecdotal? Sure, but this is a pretty unique case. This guy has probably done more urban riding than most everyone, he rides day and night, in fair weather and foul, and he never wears a helmet. Yet, for decades now, he has somehow managed to avoid the supposed impending doom that awaits non-helmeted riders. I mean, can anyone really be that lucky?
That's one of the things I liked about this guy: He shatters expectations, but he does so in the most unintentional and unassuming manner. All cycling talk aside, it's a funny and entertaining doc. If you like stories about quirky and interesting people, check it out.