I have nothing against wearing helmets but neither do I view people who make an informed decision not to wear a helmet, as foolish risk takers.
Helmets are classified as "secondary safety" or "passive safety" devices. Whilst "primary safety" or "active safety" involves eyesight hearing, balance, road craft, riding skill, brakes, tyres, wheels & frame structure, maintenance etc. The term "primary safety" infers that these issues are related to risk management and accident avoidance whilst "secondary safety" is concerned with reducing the risk of injury when the "primary safety" fails. So the greater the risk of the "primary safety" failing the grater the need for secondary injury protection.
So an inexperienced cyclist with a badly maintained bike is best advised to take some lessons and get their bike fixed before they ride at all, with or without a helmet. The big danger is when people believe that the helmet on its own, will make them a safe rider.
It used to be that the British Standard for the old hard shell helmets was to be able to protect against brain injury only sub 10mph impacts. I don't know what the current standards are but 10mph is not enough protection to be able to rely on. In certain categories of accident helmets have been shown to exacerbate injury.
So ride with due care and attention. And in order to protect you from other road users wear a helmet. But don't expect it to save you if you or they, mess up.
Off road. If you ride well within your limits and practice how to recover or jump clear if things go wrong, then a helmet is not essential. In rain a hat that keeps the rain out of your eyes may in fact be preferable.
You can find many more arguments and research both pro and against here:http://en.wikipedia.org/wiki/Bicycle_helmet
9/16" Suntour BMX MP-1000 pedals / 56 tooth TA 6-bolt chainring.(I'm not bothered if the teeth are worn or damaged)