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

The counter-argument to that is that It's UCI regulations and they have to keep the sport appearing safe to the public. Can you imagine the outcry if someone crached and died when not wearing a helmet as opposed to the fuss if someone was wearing a helmet?

The immediate publicly-given reason why helmet compulsion was introduced was the death of Andrei Kivilev in 2003. He wasn't wearing a helmet when he crashed in a freak incident, touching wheels with the rider in front while not having his hands on the bars.

Interestingly, riders are allowed to remove their helmets on climbs, but I assume they would have to put them back on before the descent. Its a while since I read the rules, so I may be wrong...

No, the exemption to ditch the helmet on the last climb up to a summit finish was removed some time ago.