I think that pretty much sums up my approach - I always wear a helmet mountain biking for the simple reason that falls are mainly (for me, anyway) low speed where a helmet might actually do some good. Likewise, when learning to ride my recumbent bike, I wore a helmet for the first few weeks as I had a tendency to fall sideways while stationary so there was always the possibility that I'd clonk by head on a pavement or road furniture (although helmet wearers, please note, helmets are only any good at absorbing shock of hitting a flat surface and invariably fail when hitting a shaped object due mainly to the air vents). I often wear a helmet when I going to be around loads of other riders of unknown skill level - my only serious injuries have come from an idiot amateur racer (on a bike) taking out my front wheel in a badly judged overtaking manoevre in a mass start race (and still I didn't hit my head).
On the road, all my big accidents have involved cars/trucks and not my head (plus, I should add, I have a long history of parachuting - before the days of standing landings when learning a 'para roll' was de rigeur, climbing, bouldering, martial arts etc. all of which tend to emphasise knowing how to fall) so although I acknowledge that a helmet might save me some superficial wounds in the event of, say, clipping the pavement with a pedal a la Lance, I am confident that it wouldn't do anything to 'save my life' in the event of a collision with a car.
So it's thousands of hours of lidless comfort and possibly minor wounds v. thousands of hours of sweating under the helmet, finding somewhere to put it in the cinema/restaurant/meeting, trying not to drop it (they're so flimsy dropping them will damage them to a state of uselessness - expensive) and then what - saves me a couple of stitiches - no deal
