About two years ago, I attended a funeral for a co-worker who was riding w/o a helmet and was hit by a car that turned into her. She died of massive head injuries. Yes, the accident could have been prevented if the driver had been paying attention. But a helmet may have saved her, and as we all know, many motorists are startlingly oblivious to bicycles and those riding them. We might as well avail ourselves of what defenses we have.
I have not ridden w/o a helmet since, except in a few instances where I've been making adjustments/repairs on a bike, and riden the bike up and down the street I live on to check said adjustments and repairs.
I'd also like to limit the number of funerals I attend, esp. those for twenty-one year olds. That said, it's your choice. But I've found a graphic reminder does serve to focus your attention. Short of cars dissapearing from the earth, I'll be wearing a helmet.