I've always worn a helmet, but have still been sort of sympathetic to those who say that it's not necessary and won't necessarily help you that much in an accident. Maybe. Depends on the accident.
A couple weeks ago I slid on some black ice making a turn onto the street where I live and BAM, pavement-kissing time. It had been rainy that day and the night temperatures had fallen just below freezing, making it prime weather for unexpected ice.
This poor helmet's been retired now after a job well-done. Note the crack in the foam and the road-imprint above it. I had a headache the next day, but was back up to 100% normal within 36 hours. One of those things that could have been way worse.
If I wasn't a helmet evangelist before, I am now.