I've only ever hit my head once in 10 years and it was only a minor knock, laughed it off.
I have never worn a helmet riding, not for any other reason other than I just don't like the feel of them. You learn how to fall and you think hard about what you're doing before you do it. Those two things are most important.
There are some accidents that occur where you really have no chance to protect yourself and they are really unfortunate but it has never been enough for me to want to wear mine.
That being said, I ride with some 15 year olds that wear their helmet when we ride street, I don't say a word to them. If they wanna protect their head who the fuck am I to tell them otherwise.