There have been lots of discussions and arguments about bicycle helmets, even though the subject is quite simple. A helmet is a passive safety equipment that can mitigate some consequences of a crash, but doesn't prevent the crash itself. So by having a helmet on your head, you're not automatically guaranteed to be safe.
Of course, it has some safety effects in case of a headfirst fall:
- It provides a layer of relatively soft material between your head and sharp objects.
- It provides sort of crumple zone. Styrofoam is not very crumplable, but still better than nothing.
- Smooth plastic surface slides well on asphalt, so the passing-by road surface shouldn't catch your head and break your neck.
- If you don't plan to crash, you can use you helmet as a platform to mount active safety measures like reflectors, lights or mirrors.
- Nobody shouts "Where's your helmet, ******?" at you.
There are also some disadvantages:
- According to this study, drivers pass helmeted cyclists by about 8 cm closer than those without helmets. Maybe they look more experienced or less vulnerable to them, anyway this is a statistically proven fact, not a thought experiment. So with a helmet, you have greater chance to survive getting hit, but also greater chance of getting hit in the first place.
- You can die of many other injuries than just a head blow and a helmet doesn't protect you from them.
- Cyclist with a helmet can feel less vulnerable and ride more risky (but that's his own problem).
- A helmet can get caught by something that wouldn't catch on a bare head or would miss it (quite improbable though).
- A helmet is impractical, it's another bulky item you have to carry, it strains your neck, mangles your haircut, interferes with hats etc..
Some racers consider occasional crashing to be a normal part of life, because those who don't crash don't push their limits far enough. These wouldn't live long without helmets. With a standard self-preservation instinct, on the other hand, riding a bike isn't any more dangerous than any other everyday activity; much more head injuries occur when going by car or just walking.
Every now and then, someone gets an idea that helmets should be mandatory for all cyclists. I'd recommend to look at Australia where they already made it real. Some bikers bought helmets, but others quit biking, so there are now less bikes on the roads (by some 30 %), drivers are more surprised when they meet them and accident probability is higher. The result: there are more accidents now than before the mandatory helmet law and head injury rate didn't drop significantly (source).
Decide for yourself.