I did quite a lot of reading up on cycle helmets when I got my Agattu... I stumbled across that cyclehelmets.org site whilst I was trying to find a nice looking helmet and read pretty much everything on there, rather surprised by what I was reading - I was expecting it to be a cut & dry thing that helmets would inheritantly be safer.
Having read all of that site and a few more, it is now my understanding that the following are all contributing factors to cycle helmet safety...
The most damaging kinds (i.e. fatal) of head injuries are ones where the neck is twisted. A sharp blow to the head isn't actually that bad as the brain is well protected by the skull, and unless the skull is actually fractured then you'll most likely be OK. You scalp is actually able to move independantly of your skull and acts as a slippery layer which allows your skull to move underneath it, and this offers it a surprising amount of protection from angled impacts (such as banging your head on the kerb).
Cycle hemlets strap under your chin and increase the size the size of your head to by a couple of inches. This means that if you should catch your helmet as you fall then you are far more likely to suffer a neck-twisting injury than you would otherwise be without a helmet as your head has a larger diameter and any angular impact will have more of a twisting action on the neck. Wearing a cycle helmet makes you feel protected, which in-turn has a makes you inclined to go faster, but cycle hemlets are not really designed to protect you for any kind of high speed collision. If you go at 30mph headfirst into a wall/tree, your helmet is toast, and most likely, so is your head.
Given the head-diameter/nick-twist factor, there's a fair chance that a wooly hat is the safest of all, but nobody's done a proper study of that yet.
Most cycling fatalities are car-related, and in no small part related to how cars expect to behave on the roads. The more cyclists on the road, the more cycle-aware drivers are and the less likely an accident becomes. I believe that Australia had this happen after they introduced cycle helmet laws. The helmet law caused a reduction in the number of cyclists, which sort-of in turn cause an increase in the number of fatalities as motorists began to behave differently.
So.. to conclude this rather long winded post... my opinion is now as follows.
The healthy exercise you get on a bike significantly decreases your chance of an early death regardless of the accident risk. The more cyclists on the road, the safer it becomes. If wearing a helmet is going to make it less likely you'll cycle, then don't wear one. If wearing a helmet makes you more likely to cycle then wear one.
Personally, I go helmet free.
I took your main point and demolished it.
