Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

Whatever your 'risk assessment' of the situation is and however 'excellent' your bike handling skills, the unexpected may still happen
Imagine having to live the rest of your life in a wheelchair, knowing that you could have just put that helmet on... Or even worse, imagine how your family would feel if they knew you'd still be with them if you'd just put it on.

True no one can expect the unexpected and depending how risk averse you are will depend on the degree of protection you take, however cycling is a relatively safe activity marginally more risky than sitting on a bus per mile travlled. Drivers who crash often suffer head injuries. By your logic drivers (or pedestrians for that matter in case they get knocked over crossing a road) should wear a helmet just in case, as should pub goers, hill walkers, people in kitchens...

In fact i have no issue with helmets and anyone wishing to protect their head should consider wearing one. Problem with cycle helmets is that many people (especially non cyclists) default to the pro helmet position because it seems common sense. That default discourages some people from cycling thinking it is much riskier than it is. Not cycling is so much riskier