Cycling is about six times more likely to kill you, per hour, than driving. So while helmets in cars would help some, the case for them on bikes is stronger.
It isn't about removing all risk - some danger is inherent in pretty much everything you do. The point is to manage the risk, and take acceptable precautions where they are convenient.
I've always worn a helmet when cycling -- and I'll continue to do so -- but I've never had an accident in which it mattered, and I don't think it changes the odds as much as you suggest.
There seems to be a strong opinion in these comments that because a bike helmet isn't a magical force field that prevents any possible injury, you shouldn't wear one (with the implication that because helmets don't protect you from every possible injury, you shouldn't ride a bike). All i'm trying to say is that helmets are better than no helmets, and no matter what safety precautions you take, risk is inherent in everything we do.
Btw, would you also wear your cycling helmet while walking on the pavement as a pedestrian? At least in the Netherlands there are more head injuries for pedestrians that fall or hit something than cycling-related head injuries. :-)
I think helmets are associated with bikes, because before we used bikes to commute, which involves simply riding about town, we used bikes to rip through the woods inches away from trees and sharp rocks, or jumped them off of mounds of dirt, or flung road bikes down mountain roads at 50per. (at least those three seem to sum up my first 23 years of existence) I live in DC, got old, but still NEVER commute with a helmet, and feel perfectly safe all the time. In fact, tons more safe than when I try to get a training ride in. (for which I wear a helmet)
Around here in Santa Cruz, a town known for biking -- many local builders, a couple of clubs, and a very forceful pro-bike lobby -- we still don't have the infrastructure that places like Portland or Amsterdam have. In fact, the roads are pretty much crap in most places. So you have to be careful. One recent solo accident involved a young adult who was biking at night on a dark road (dumb) but also not wearing a helmet. He crashed. Currently he is in critical condition. Pretty sure a helmet (and a light) would have helped him out.
The story -- http://www.santacruzsentinel.com/localnews/ci_21655013/santa...
Personally, I've had some spills while mountain biking where a helmet has come in pretty darn handy.
I don't ride much, but one time I was out is the rain and had to swerve on a intersection that happened to be made of cobblestone, and ended up going down hard on the slick stone. Had I not been wearing a helmet, I would have had a severe head trauma.
That type of accident is pretty common, and wearing the helmet avoids lots of needless suffering.
Actually: http://www.active.com/cycling/Articles/Bike_helmet_crushed__...
But, in general your point is correct. There are some dramatic examples where a bike helmet saved someones life, but statically it is unlikely.
Lots of statistics thrown around here, and my anecdotes are not statistics, but they are a heck of a lot closer to actionable information that hot air. As for me, I don't regard a helmet as making bicycling seem dangerous, any more than wearing a raincoat makes walking outside seem dangerous. Its just part of my equipment, like the water bottle.
In the bay area, there is a very large biking community -- road, mountain, around town, etc. The problem is, the infrastructure isn't there for bikes compared to Amsterdam. We have "bike lanes" that any vehicle can enter within, say, about 150' to make a turn onto another street. Vehicles don't look for bikes. The bike ways in Amsterdam, certain parts of Munich, and other towns are often separate from the roads. When not, people expect them and are courteous to them much more than here.