No one is saying you shouldn't wear them, the debate is whether or not countries should make it mandatory. If you require bicycle helmets then fewer people ride their bikes, leading to fewer people pushing for safe bicycling laws, more cars on the road, and a less active (and thus less healthy) population.
You're correct about what the evidence actually shows. But unfortunately quite a few people misinterpret it and end up saying that no one should wear a helmet at all, even voluntarily.
See for example this stupid article, which was originally titled "The Case for Ditching Your Helmet." I came across it because a friend posted it to Facebook as "proof" that wearing a helmet never made anyone safer.
But this isn't shown by the article. Cycling dropped, but there is no indication that this was caused by helmet laws. A much more detailed study would be required to determine the cause of reduced cycling.
Well, except that the same thing has been found in other studies as well. The results of this study are completely unsurprising to anyone who has read any other study of helmet laws in his/her life.
At some point you just have to accept the evidence.
It's not about accepting the evidence. It's about making sure we interpret it correctly.
The main question to ask is this: why exactly do mandatory helmet laws reduce bicycle usage, and what needs to be done to address it?
Is it that helmets are too expensive? (They aren't.)
Is it that they are too difficult to put on and take off? (They aren't.)
Is it that they are too uncomfortable? (Maybe, but not any more so than seat belts, and yet most people aren't arguing against seat belt laws.)
Essentially, I'm not against accepting the validity of the evidence from these studies. I'm simply against the mentality of "mandatory helmet laws reduce bicycle usage, therefore we must not have mandatory helmet laws." That conclusion is too simplistic. The topic requires further study.
Helmets are quite inconvenient. One's head gets sweaty, one's hair gets mussed, one can't hear anything for the wind noise, and seeing a helmet gives autocage drivers the psychological permission to leave about eight inches when passing. Those who chose to cycle without a helmet have their reasons, just like most other human beings who do as they choose.
Is it that they are too difficult to put on and take off? (They aren't.)
I wouldn't assume that. As far as I can remember, this is one of the main reasons cited for the reduction in cycling. Small things can change a persons behaviour significantly.
They're uncomfortable and inconvenient to keep with you once you're at your destination.
There is also potentially (depending on which studies you give credence) the danger of risk compensation causing helmeted cyclists to cycle more dangerously and automobile drivers to pass more closely to helmeted cyclists.
Obviously not scientific, but if you need some totally informal causative bro-science to allow you to swallow these empirical results:
Even if the benefits are "obvious", the large majority of people aren't self-improvement-focused supermen with significant reserves of willpower at their disposal. Even the smallest increase in the necessary "activation energy" (comfort, expense, convenience; whatever) necessary for a task is going to put a large chunk of the distribution on the "fuck it, I'll just drive" side of things.
This is why "engagement" and "user experience" are multibillion dollar industries. Compare abstinence-only education, condom usage, and (kinda, if you squint a bit) drug prohibition.
Ideally, you would want some survey data, although I think it might be hard to get people to say that they stopped riding their bike because wearing a helmet is inconvenient, even if it's true.
I don't think the argument is if bicycle helmets are good or bad, but rather bicycle helmet laws. You can be for wearing a helmet at all times and still against bicycle helmet laws. Much the same way you can be for the legalization of drugs but not be a drug user yourself.
I wouldn't try to convince you they're a bad idea, although I might try to convince you that they're just as good of an idea for pedestrians and people in automobiles, where they would almost certainly also save lives.
Gov't action to support bicycle helmets are a good thing - the point is that action to forbid non-helmeted biking turns out to be bad, as the main effect is not to nudge people from biking w/o helmet to biking with helmet, but to nudge people to not biking at all.
You are guided by faith, not data. You wore a helmet when you crashed, and you lived. You also wore socks and underwear.
EDIT: I realize that there is no debate, as I will "never convince (you) they're a bad idea." I'm talking to the people who are coming at this with an open mind.
The only time I've ever crashed on bicycle was the one time I wore a helmet. I have stopped wearing a helmet because of the same 'logic'. I use my bike to get to work.
One minute from now, someone is going to hit you in the head. You have the choice of whether to put on a helmet first, or not. Which would you choose? Thus the benefit of wearing a helmet is demonstrated.
