Here in the Netherlands almost no one wears a helmet on a bicycle. There have been studies that suggest cars are less careful with cyclists who wear a helmet.
Where I am, there are more things than cars that will take me out, and a helmet is one thing that protects me.
Low hanging or falling branches, massive potholes that can kill your tires and bike wheel, people carrying large objects out of the trunk of their car - so even if they are driving safely, it only takes one snapped bungie cord to have a piece of furniture come at you...
Always wear a helmet regardless of how safe you think cars are being!
According to http://www.bicycling.com/news/advocacy/ride-smart almost half of car-bike accidents are due to cars turning left, and NOT seeing the cyclist.
There are many, many places where the option to ride without a helmet does wonders for cycling culture.
I'm genuinely interested in the benefits of forgoing safety equipment that is readily available, easy to use, and proven to save lives.
Perhaps my experience is skewed because I almost exclusively do mountain biking. You couldn't pay me enough to ride on urban streets contending with cars.
Same rule applies for driving, walking, and basically everything.
There are several videos of cycling in the Netherlands on YouTube. Most of them have been speeded up because they look so slow.
Helmets are pretty universally known to reduce traumatic brain injuries.
Elvik writes "When the risk of injury to head, face or neck is viewed as a whole, bicycle helmets do provide a small protective effect. This effect is evident only in older studies. New studies, summarised by a random-effects model of analysis, indicate no net protective effect".
I'm guessing you didn't read it and assumed it supported your preconceptions?
I specifically mentioned head injuries (TBIs) - there are two meta analyses listed, the point you listed went beyond that (to include neck and face injuries). The other meta analysis also supported its TBI-reducing benefits.
Edit: If you're looking for the pertinent line from the analysis:
"With respect to head injury, the answer is clearly yes, and the re-analysis of the meta-analysis reported by Attewell et al. (2001) in this paper has not changed this answer."
What you actually said was, "Helmets are pretty universally known to reduce traumatic brain injuries."
This is wrong and not what you are now saying. You are now saying that among people who have suffered brain injuries, helmets are known to reduce traumatic brain injuries.
If the helmets make it more likely that you suffer an injury in the first place, it's not really a consolation that they mitigate the effects. There are actually reasons why many people oppose compulsory helmet laws.