Having just gone through the learning to walk stage with my son, I'd say they are a complete waste of money. DS had a few tumbles in the first few weeks but not often did he hit his head, more so falling on his bum! I agree that kids need to have a few knocks to learn to be a little cautious and the helmet could give them a false sense of security, so what happens when it comes time to lose the helmet??
Anyway, just my opinion