You are not going to be able to use statistics to prove this either way. The population is just not big enough to measure real results. If you want to do this then go to China and make everyone wear helmets, wait 5 years and see if the injury rate has fallen. So lacking any way to measure statistically you rely on common sense and anecdotal evidence. Common sense says that wearing a helmet will reduce pain and injury if I hit my head, this is true walking, riding a bike or mowing the lawn. Riding a bike you are moving at a higher speed so it may have greater effect.

Riding a bike is pretty safe compared to most things we do so I don't care whether you wear a helmet or not, really this is not driving up insurance rates. I wear one since I personally feel I have a better chance hitting my head riding than doing other activities and why not. You make your own decision, let others decide for themselves and quite using bogus statistics on either side to convince others of your position.