For a typical non-heavily-trafficed-commute or a weekend ride-in-the-park - there's very few accidents.
When there's an accident, the helmet makes a difference.
"It seems intuitive that bicycle helmets would be an effective means of reducing head injury, and indeed helmet manufacturers and standards associations worldwide conduct rigorous tests to determine the impact absorption and other qualities of helmets. It is however, important to go beyond laboratory tests and understand whether helmets reduce injuries in the event of a crash in real‐life; hence the review by Thompson et al.1 This review included five well conducted case‐control studies and found that helmets provide a 63–88% reduction in the risk of head, brain and severe brain injury for all ages of bicyclists. Helmets were found to provide equal levels of protection for crashes involving motor vehicles (69%) and crashes from all other causes (68%). Furthermore, injuries to the upper and mid facial areas were found to be reduced by 65%, although helmets did not prevent lower facial injuries. The review authors concluded that bicycle helmets are an effective means of preventing head injury."
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2598379/
And then my personal anecdata: 25 years ago I went over the bar as I hit the brakes to stop before a child that ran to the cycle path from a bush. I broke my radix but not my head - I did not have a helmet. I realized how lucky I had been. I started to wear the helmet.
Last winter I fell when I didn't notice ice on my home road. Before I could unclip, I was falling sideways. I bruised my ribs and hit my head on the ground, but avoided cracking my skull because I did have on my helmet that I've been wearing the 25 years since the previous bone breaking fall.
Don't be stupid. Wear helmet when riding bike.