I usually wear one... because it "feels" wrong not to. Like riding in a car w/o a seatbelt. I have ridden around the neighborhood occasionally without one, but only because I either forgot or i'm just cranking my pedals after oiling the chain.
We see a lot of people without them... generally Latinos commuting to/from work, and when I see a helmet-less rider I don't think twice. Most likely, they will be fine without it.
I can see some of the rebellion against them though. When I started cycling, I was told all the things I HAD TO HAVE when I bought my bike, by my hard-core cycling friends to facebook friends to even my husband. And in the end, I realized that a lot of those things, I really didn't NEED or even want. Of course I agreed that I needed a helmet, wanted a helmet, but I can see a very rebellious person rejecting even that, just to go against the grain of the HAVE TO HAVE people.
~ working mom to 3 little girls ~
Roadie... 2010 54cm Trek Madone 4.5, Bontrager inForm