Enough people have documented getting hit, on both motorcycle and bicycles, slamming their head on the ground, and having the helmet shatter or break, but their skulls end up fine. That's reason enough to wear one for me because knowing my luck, if I ended up in the same situation without a helmet, it would be my brains on the asphalt. Of course someone else would be lucky enough to not wear a helmet, get up, dust themselves off, and walk away. But I'll err on the side of caution that I'm going to be more like "Bad Luck Brian." Not everyone can be cool guys like the usual cast of characters here on CAG.
Helmets are not invisible force fields that are going to protect the rest of you or guarantee you won't get hurt, even in their head. If I get hit by a car, I don't expect the helmet to protect my body or prevent my neck from snapping. But if my head slams the road, at least the helmet will cushion that blow and prevent my brains from getting to know asphalt. For all of the insults like "Look at my new helmet and how shiny it is doesn't it match my new bike so nicely," you can flip the same douchebag mentality onto people who don't wear helmets: "Look how I'm not riding with a helmet. I'm so carefree in control of me and my shiny new bike.

everything else, I'm invincible." And with perdition(troy's attitude toward helmet wearer and his following thoughts, he admits it could save his life but he'd rather just lose it all in case he were to become a quadriplegic. I'd rather wear the helmet, see if it helps/assists in keeps me alive, see what my injuries are, and if I don't want to live I'll take care of the rest later. If you don't want to wear a helmet, great, that's your call. I don't mock anyone for it. Your life, your risk, your decision. I don't know why non-helmet riders have such a hard on for going after helmet riders. It probably does have to do with the mentality of "no one can tell me to wear a helmet anymore.

you for choosing to wear one."
And as for comparing the US to European countries, that doesn't even make sense. The US is so much bigger and more diverse than many European countries in terms of city structures, the number of autos on the roads, the availability of bike lanes, urban vs. suburban vs. rural, etc. when you're just talking about commuter biking. It would make more sense if people compared similar situations, like say, New York City vs. London (probably just central London and not the suburbs) instead of USA vs. UK. I wouldn't even consider NYC vs. LA a good comparison just because the cities couldn't be more different aside from being populous cities.
I'm actually shocked to hear you motorcyclists say you won't wear a helmet. I'm actually shocked that is even an option for you. In my state, you have to wear a helmet while riding. The only times I've seen people not wear a helmet is when they were just test riding a buddy's motorcycle in a residential neighborhood.