Looking up there, I agree that not wearing a helmet is just plain stupid when you look at the statistics. However, I do agree with Munch as well, that sometimes you need to let people make decisions for themselves. Also, something Munch may agree with too, a coworker of mine often says that we are trying too hard to keep stupid people alive.
Helmets do not always make people safer. Safety gear itself does not always make people safer, same with mandating it. There are a number of factors at play here. One is that sometimes people are unfamiliar with the safety gear and will use it improperly and perhaps put themselves at a greater risk because of it (not likely with helmets, but the point is valid). Another is that some people with intentionally use the equipment improperly, I know of a person that still refuses to put on their seat belt, they will go so far as to stretch it across their chest and tuck it under their butt. There are always going to be some people that will simply not use the equipment, mandatory or not. The one reason that many people tend to forget is that when we feel safe, we tend to take greater risks. It is not a hard-fast rule, but going atgatt can (not will) make you stupid, thinking going along the lines of “I have my gear on, I’m safe, now watch this wheelie” (a little hyperbole, I know).
Now, the one of the top safety tips I hear/read (other than atgatt and rider training) is make yourself as visible as possible. I don’t know about anywhere else, but here in Oregon the easiest way to make yourself visible on a bike is to have a light-colored helmet; it even seems to be more effective than a light colored jacket.
I like the PA law that WeaponZero was talking about, having new riders wear helmets and letting more experienced riders choose. It lets you have your freedom, but keeps new riders protected while they are most likely to have an incident. A decent balance.