Happened to be in a urban area. Happened that a car cut the way and i hit it, and the ground with the head. I have a permanent scar (its not so bad) on the side of my face.
Without helmet it'd have something much worse (yeah i broke other body parts, the hit was rather violent - i can avoid a car going at regular speed, but if i'm going fast and the car is speeding, sometimes it's though luck). So you know what? As long as I'm not going down at 60mph in the forest I don't care if I've a helmet or not. Ground is soft, and I'm probably not gonna fall. The chance to hurt my head is low, and the skull is strong.
Now in a urban area, 60mph isn't unheard of. Specially from cars, even if you're at "only" 20mph on the bike. Thats exactly where a helmet is a good idea.
For sure I'm ok with people choosing to wear a helmet or not (as long as if a biker gets hurt by it's own fault, the car driver doesn't have to pay for it, which I fear ain't the case in the US). But I don't think that's a very good idea in a urban environment.
You'd need bike-only lanes that are fully respected, proper visibility, etc. That's not the case in Paris (velib) and that's not the case in the US. In both cities bikers also pass at red light, in both cities non-bicycles use the bicycle lane (in Paris scooters take it all day long to avoid traffic, at high speed). In some other EU countries, or some other French cities it's (much) better than in Paris, though, safety-wise.
Also in the EU (Minus Paris and probably a few other big cities), cars will not attempt to kill bikes if there's no bike lane, and bikes will generally respect the road signs. Makes a world of a difference.