I think there are two types of riding a bicycle, commuting/recreational and sport riding. There is no need at all for a helmet for simple commuting as the speed is mostly between 10-20 kph. Wearing a helmet at those speeds will add absolutely nothing in safety. Most accidents that happen there, in the city, are collisions with cars, and the helmet won't do jack for you.
But for sport riding the speeds go up to 30-40 kph or even more, and that's where a helmet is essential. Most accidents that happen to sport riders are single sided, like slipping, or a collision with a steady object, and that's where you can get seriously damaged.
Here in Holland everybody has had a bike since we could walk, and I rode my city bike for 40 years without any serious accident or injury. And then I got a faster touring bike with 3x8 gears, and suddenly 40 kph was a reality. I had the bike for less than a year, doing 30 kph, when I slipped on a wet spot on a bridge and broke my clavicle and bruised my head. Head injury was nothing serious, luckily.
Nevertheless, in the hospital they very thoroughly checked my head for internal bleeding, as that can suddenly kill you overnight. When I got home I saw in the news that happened the same day Michael Schumacher hit his head while skiiing, and became a vegetable. I decided that from now on, I needed a helmet with these speeds. Funny thing was, I was 35 km from home when I fell, later I bought a very professional helmet for peanuts from a guy living only 500 meters from that spot. Some things are just meant to be.
Coen, from here and previous posts of you I sense a lot of mistrust towards car drivers from you. I do not share your sentiment, at least not here in Holland. In my experience, cars are always very careful around cyclists, except maybe the occasional 21 year old in an overly tuned Japanese car. On the other hand, I see a lot of dumb things from cyclists, especially those on a racing bike, as they think they own the entire road, speed by in crowded spots, and ignore traffic lights as they please.