At least one study shows pretty conclusively that in almost all "no car" bicycle accidents and in a very large percentage of car-bicycle accidents, head injury was about the only injury that was likely to be life threatening or permanently life-altering. I.e., even when hit by a car if you can reduce or eliminate the head injury you have a pretty good chance of not only surviving but of eventually making a full recovery.

A lot of folks don't wear a helmet because 1) off the street they figure they're safe and 2) on the street they figure that a helmet isn't going to be enough to help if they are hit by a car.

Studies both in the US and Australia are pretty conclusive that both assumptions are incorrect. Head injuries in "bike only" accidents are more common than people think. In the majority of car-bike fatalities the fatality results strictly from head injury (i.e. other injuries are usually sustained in the same accident but they are usually not life threatening) and even a lightweight bike helmet can significantly reduce the severity of injury in a car-bike accident.

In the "typical" car bike accident the car hits the bicyclist, sometimes breaking his legs and often throwing him upwards and "spinning" him feet up, head down, just about the time that the car windshield, moving much faster than the cyclist, reaches his head.

BTW, when I lived in the high desert of SOCAL I arrived on the scene moments after just such an accident in Victorville CA - I was the first on the scene, in fact. I was on my way home with pizzas for the family, driving uphill on a 4-lane non-divided rural street (speed limit 45 or 50MPH) into a completely blinding sun. As I neared the top of the hill I saw a car stopped in the right lane, as I got closer I could see that the windshield was shattered. As I got nearer still I saw a BMX type bike under the car. I pulled over and there was no one around, then I found a black teenager, probably about 15 or 16, in the weeds a few yards off the road. A few moments later the driver showed up, she'd gone for help from some nearby homes across the road.

I stayed with the kid and kept the idiots off of him until the paramedics arrived -- well meaning but very stupid people kept trying to lift his head and put a pillow under it to "make him more comfortable." Others started to manhandle his legs asking if he could move them. I was flabbergasted that people could be that stupid! I think I was still repeating "head and neck injuries, don't move him" when the paramedics showed up. I know I remember one of them nodding at me and mouthing "thanks" as I moved out of their way.

He was coming around by the time the paramedics arrived but he was bleeding from nose, mouth, and ears and it was pretty obvious that he had a very serious head injury. He was not wearing a helmet. If he had been, he probably would have been only lightly injured as the woman who hit him (after he darted right out in front of her from the desert scrub off the side of the road) was only going about 35MPH when she hit him and it was clear that even his legs hadn't been badly damaged - all of the visible damage was to his head.

We didn't take the local paper and I never did find out what happened to that kid - but any time I wonder if it's worth putting a helmet on for a short spin around the block I think about that kid and the blood coming out of his ears. At the time I hadn't been riding for a while, and I'd never used a helmet before when I rode. When I started riding again I bought a helmet even before I bought a bike. Granted, that was partly because my cardiologist was adamant about the dangers of even minor knocks to the head with the medication I'm on, but even if not on blood thinners I think the image of that kid would be enough.

I'm not an "evangelist" for helmets - as far as I'm concerned if you want your family to spend the rest of their lives changing your diapers and spoon feeding you your mush that's entirely your business. On the other hand, before making that decision you might want to wade through this report that summarizes (albeit not briefly) the results of studies of real-world cases:
http://www.helmets.org/henderso.htm

John