Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

Pretty much everything is inherantly risky and I would argue that cycling is one of them... you are always some diesel, or a stupid ped or a stuck cleat away from hitting the deck in a way that is much harder to break your fall than as a ped.

Yes, when I see those videos of people in their suits or skirts pootling to work on the segregated cycle lanes of Amsterdam and Copenhagen I can't help but think that they're a mere hair's breadth from a nasty tumble!

Yes, there are much more important things than helmets. Yes, they will do little or nothing in many serious accidents. But do either of these things make wearing a helmet irrational, harmful etc etc?
Not wearing a helmet, no. But they do make encouraging people to wear helmets irrational, because every bit of encouragement, urging, pleading, emotional blackmailing and outright legal compulsion (and I put these all into the same category) spreads the general feeling of "well, if all cyclists wore helmets there wouldn't be a problem". Now we all know this is bollocks, the risks will be very similar (and possibly worse), but we don't get to make the decisions. Scantly-informed and vote-hungry politicians under pressure from the even less informed public and the knee-(and tear-) jerking press get to make the rules.

Helmet encouragement/campaigning/compulsion is, at best, a distraction from and, at worst, actively leads us away from the meaningful investment in infrastructure and the legal and cultural changes needed to make mass cycling a reality in the UK.

Edit: I'd like make clear that I wear a helmet and I would never criticise someone else for doing so. I also think there may be a reasonable case for making very young kids wear helmets up until an age that their risk of falling simply from cycling along is reduced.