Sure, wearing a helmet while crashing at 60mph head first into a concrete block may not stop some serious damage, but surely common sense must say that if there is a layer of protection between your skull and a hard object, your skull will stand a much better chance of not cracking.
I can completely understand why people don't want to wear a helmet - it might mess with their hair, or ruin their cool factor, or because all the pros never used to wear one, then they shouldn't have to. Really, there's no justifiable reason that any of you naysayers can produce for NOT wearing a helmet. There's no benefit from not wearing it. Most decent helmets have enough ventilation, you can clip it to your backpack or just carry it when you ride to the corner store, you've got that extra layer of protection, and if for some reason you have a slow speed fall (i've seen more than one cyclist fall while track standing and they've ALL smacked their head on the ground), that protection will come in handy.
More power to those who don't wear a helmet though - i REALLY hope you never have a situation where it would help. That though, would only serve to provide evidence to your argument, because you would interpret your lack of crashes as reason to not wear a helmet.