My father was a bike commuter for many, many years, and suffered a very significant head injury after being struck from behind by a car while riding home from work one Summer afternoon. If it wasn't for the fact that he was wearing a good helmet, his life would have ended right then and there.
As I've talked about in other TB cycling threads, I spent more than 10 years racing bicycles, which included many thousands of miles sharing the road with cars during training rides. I had two occasions where I walked away with a from a broken bike and a shattered helmet with only a mild headache. One incident involved a car and the other was a mechanical issue that lead to me flipping at 30+ mph. In both cases, my helmet did its job perfectly, and I was happy to shell out the money for a replacement.
I have too many other stories of friends who have been saved from serious injury, or death, because they were wearing a helmet. I'm talking about all very skilled riders here -some of them spectacular bike handlers. Unpredictable things happen on the road, and no matter how good a rider you are, you can't count on car drivers to always see you, and others just don't give a damn. An 18 lb. bike is no match for a 3000 lb. car - ever! Protecting your head with a helmet is easy. Recovering from a brain injury is not. If your justification is "too hot/bulky/annoying", that's a very lame excuse.
As far as legal mandate, I'm honestly not sure where I stand (for adults - kids are a different issue). My own personal policies on the subject are enough for me. If I'm on the streets, I'm wearing a helmet. If my kids are on a bike - even in our driveway - they're wearing a helmet.
If you choose to put your own grey matter at risk by not wearing a helmet, that's your choice, I guess.
"Stupid is as stupid does" - Forrest Gump