Many people (riders or not) in the US have a zero-sum kind of attitude towards bike helmets - helmet good, no helmet stupid. I think the reality for slow, urban riding is far more complex and frankly there's a fine case for going without.
In case you missed it (as I did), here's an articulate post on the topic by James Schwartz over on The Urban Country.
To which I would only add: yeah, what he said. I personally don't wear a helmet, and James does a great job explaining the rationale. If you wear a helmet, that's great if that suits you, power on.