I've also seen the argument advanced that requiring helmets makes cycling less safe: Some people will refuse to wear them and won't bike at all, and others will get the impression that cycling is more dangerous than it is because a helmet is being required. (Are drivers or pedestrians nagged to wear helmets? Besides, the most common injury from cycling isn't a head injury - it's a broken wrist from trying to stop a fall.)

The decrease in cyclists on the road reduces the likelihood that drivers know how to share the road with them, that cyclists will learn from each other how to ride safely in traffic, etc.