OK, I'll bite… I don't wear a helmet for local XC rides. It's a matter of risk assessment: I know the trails extremely well, I 'know' exactly what my speed will be and that I won't crash… Of course, a branch might fall just as I pass beneath but the chances of that are infinitesimal. And I enjoy riding helmet-less.
If I ride somewhere new, or on faster, rockier trails (such as a trail centre) I always wear a helmet. If I go downhilling, I wear a full-face helmet. But for pootling about Hampshire? No. And I like the fact I have the freedom to make my own mind up.
As a digression, there was a very interesting article last year by Malcolm Gladwell (I think) on the severity and extent of brain injuries suffered by American footballers as helmet technology 'improved', thus enabling many more and harder impacts to be taken. The study he quoted said that pretty much any player with an impact-intensive role will have damaged their brain by their early twenties. I wonder how our rugby players are doing now it's a bigger, faster, harder sort of game.