I started cycling more than 50 years ago, so wearing a helmet wasn't natural to me. When I began to train and doing group rides, about 22 years ago, wore a helmet occasionally, but never for commuting. Now my commuting makes for 60% of my kms and more of my cycling time, so I thought I couldn't be so irrational and began wearing a helmet full time a few months ago. Now I feel strange, almost naked without a helmet. Still, I think it's a personal decision and don't look down upon those who don't wear them.
A week ago I was on a group ride, riding two abreast but nowhere near the middle of the street, when a distracted driver touched my handlebar with her mirror. I fell on my elbow, side and knee, luckily no serious consequences although I'm still in some pain. I hoped I could tell one of those dramatic "my helmet saved my life" stories, but it never touched the pavement! Frustrating.