Bikes themselves have almost no retail value; they are stolen despite this because it is so easy to steal them and because the consequences are largely non-existent. A helmet wouldn't last 30 minutes hanging from your bike in a larger/crummier city than Portland; people would steal it just because they could.
This simply isn't true; just try comparing ebay searches for "used bike" vs "used bike helmet."
Anecdotally, in the DC area people lock their helmets to their bikes all the time. I've had one bike stolen here but never a helmet. In fact when they stole my bike, they left the helmet that was locked up with it.
I think it is telling that I never see helmets left locked to bikes in public. Even if it is as safe as you suggest (and going by the amount of other worthless things that have been stolen off my bike, it is not...), the fact that the general public does not perceive it as such is what is relevant to the discussion.
