Can a helmet save your life on a motorcycle (or bicycle, for that matter)? You bet.
Will is always save your life? No way.
My biggest problem with bikes and bikes (in other words, both kinds) is not MY riding but the other idjits on the road.
In the past few years I’ve taken to wearing a bicycle helmet. My husband thinks this is nutty, and some of his arguments make sense - the fiberglass and styrofoam ain’t going to do squat if I’m run over by a steel-hauling truck, or even some of the big SUV’s and pickups around here. Truthfully, I’m not too sure how well I’d do in a full size car in scenarios like that. What I think the darn bike helmet is for is to keep my from one of those annoying brain injuries that come when you have to bail off your bike to keep from getting run over and hit your head landing in the roadside ditch, whacking into a tree, and so forth. You know - the dumb reasons for becoming permanently injured.
I think helmets (of any sort) give some folks a false sense of security. They will not prevent all death and brain injuries. The only thing the do is decrease the total number of such injuries and, in some cases, make an open-casket funeral possible. It is also quite possible to wear a helmet, protect your brain, and be left with a permanently wrecked and useless body.
When I ride the back of motorcycles (not being an owner/driver of such) I wear a helmet. It’s not just the other nuts on the road in that case, but also the higher speed. Falling off anything at 40 or 50 mph is going to cause more damage than falling off at 15 or 20 mph. Just basic physics. The only regret I have is that my motorcycle helmet (yeah, I own one even if I don’t own a bike) is not full-face - but it was the only one that fit me that was in stock in the store that day and I though it better than no helmet at all.
For that matter, I wear a helmet when I ride horses, too. I have these wonderful recollections of being bucked off a horse and my trajectory ending with me crashing head-first into the side of a barn. No injury (well, bruised a few limbs). That’s all it took to convinced me.
And sometimes I wear one when flying airplanes, especially ultralights. Too much possibility of crap getting kicked up from the ground on take-off or landing, and there’s a surprising number of bugs to slam into at altitutde. Same reason I wear a seat belt in an airplane - no, it won’t save your life every time, but it puts the odds more in your favor.
All that said - I’d prefer the government stay out of dictating safety equipment. It’s too easy to adopt a “one-size-fits-all” approach (look at the combination of mandatory airbags and small children - that was messy). And there’s no allowance for exceptional circumstances. As an example - car seat belt laws are written with no exceptions - you need to be belted in at all times. On the other hand, aviation regs dictate the seat belt is on while on the ground, taking off, or landing unless they interfere with necessary procedures required for the safety of the flight. Here’s the examples: in a car, if a kid riding in the back seat gets loose and opens a door while you’re going down the road I’d say it’s a good time for the right-seat front passenger to unstrap and save the rug-rat - but strictly speaking that’s illegal. On the other hand, in an airplane, if I’m flying along and the door pops open it would be legal for me to unhook the belts, lean over, and pull it closed IF I felt that doing so was a reasonable action to take.
And finally, yes, riding without helmets may increase the number of deaths and long-term brain injuries. But riding with helmets tends to drop the number of organs available for transplants. Which doesn’t mean we should eliminate helmets to increase organ donations of course. But it’s to point out that those saying “good of society” conveniently ignore certain statistics. Yes, “society” may pay for a few more folks’ long term care - but other lives may be saved by those whose organs now become available for the use of others. Once you consider both factors the good/bad ratio probably balances out.
And, by the way - in MOST cases insurance does NOT pay for the long-term, lifetime care of the brain-injured in the US. Unless someone has the forethought to buy long-term care insurance (doubtful in the no-helmet crowd if you ask me). And that would have to be insurance that doesn’t exclude injuries from certain activities. For instance, most insurance will not cover you for injuries suffered in small planes, especially if you’re the pilot. I’ve also seen exclusions (being in the health care industry) for hanggliding, skydiving, scuba, foreign travel, motorcycles, horses, football, gymnastics, and even cheer-leading. So read that policy folks - you might be surprised.