Wearing a helmet is always better than not wearing one. You never know what kind of accident you might be in. Sure there are some a helmet may not protect you from, but there are plenty where a helmet will. It's a stupid, pointless risk not to take such an easy precaution to reduce your odds of permanent brain injury.

Case in point, a few years I was on the home stretch of a 50 mile weekend bikeride, less than a mile from home. I was tired, dehydrated, and fuzzy-brained, but still alert enough to be paying close attention to the traffic around me. I was riding ~10-15mph in the far left lane of one-way 4-lane primary thoroughfare (35mph speed limit).

4-way intersection ahead, figured I'd cross the intersection and ride up onto the very wide sidewalk on the far side so I could get out of traffic, slow to a crawl, relax, and take it easy the rest of the way home. As I came up to the intersection I angled slightly toward the sidewalk ramp on the other side, then turned my head around to the right to watch the traffic coming up behind me to make sure no car was going to make a left turn at the intersection and either turn into me or cut me off.

All clear with cars, no left turners, but when I looked ahead again I saw I'd miss-judged my path and was going to miss the on-ramp by about 5ft and hit the nearly 1ft-high curb instead. Going too fast to correct course and hit the ramp, or to slam on the breaks and stop, only thing I could do was try jump the bike, or at least the front wheel, over the curb.

Too tired, botched the maneuver, not quite high enough, front wheel hit the curb about halfway up and went from around 10-12mph to 0 in a millisecond. Of course me and the rest of the bike kept going, me slingshotting over the handlebars (bike shoes clipped in so angular momentum at work here) and then piledriven head first into the concrete sidewalk. My head landed first with a loud crack, then my body landed on my head, then the bike on top of my body.

I might have been out for a few seconds, not sure, but the first thing I remember was thinking, "ok don't move a muscle, just lay perfectly till, you might have just broken your back and even the slightest movement could cause a tear in your spinal cord." Slowly, gradually, tentatively I started testing that individual fingers and toes worked, then hands and feet, then arms and legs, and finally body and neck.

Everything was ok, but I was still lying there not daring to try to get up. Next thought was, "ok, that loud crack was my head hitting the concrete, I probably have a concussion. gotta be careful, don't black out." Reached up to my head, started feeling around. Helmet broken in places, totalled as designed, but still attached, strapped on, conformed to my head.

Still lying on the sidewalk with bike on top of me, afraid of further injury, and feeling around my head checking for injury. No blood, no pain anywhere. Disbelief. Check again, thoroughly. Still no blood, no pain. More disbelief.

Can I really have just had a crash that bad, where my head alone absorbed the entire impact of my 200lb self and ~20lb bike, without any injury besides skinned hands and some bruises? Mindbogglingly, yes that was exactly what happened. That helmet absorbed the whole damn thing, and I walked away without either a concussion or any kind of neck or spinal injury, just some scrapes and bruises.

So yeah, just wear a helmet. Even if you have 10 accidents where a helmet is irrelevant and just 1 where it is, it will have been worth it.

If my anecdotal evidence doesn't impress you, then think of it terms of Expected Payoff E(X). Wearing a helmet costs nothing of consequence, yet the benefits could range from 0 (you're never in a crash your entire life and never need it, or you're in a crash where a helmet makes absolutely no difference) to everything (you're in a crash that would cause permanent brain damage or death without a helmet protection). You can see how the averages work out.

Humans are very poor by default at evaluating risks that have low odds of occurring, but very high consequences if they do occur. Bike crashes are exactly this kind of risk, so don't be a typical human here - don't blow these types of risks off, discount them, or ignore them. Be smart, mitigate or neutralize them.