I don't buy this. People don't realize how complicated real accidents are, so they say things like "helmets won't help if you're hit by a semi, so they're useless". Well, the first part is true. If you're hit by a semi, you're probably dead no matter what. On the other hand, if you swerve to avoid a semi and fall and tumble a bit, then hit your head on a curb, you'd rather have a helmet.

Real world traffic accidents don't usually happen because one thing goes wrong. They happen because several things go wrong at the same time.

Helmets aren't a perfect solution, but they compensate to some degree for the fragile (and unpredictably so) nature of the head.