This topic has been aired so many times, each time with respondents displaying such muddled thinking. Which is probably why it's so tempting to reply with a long Goldfish-baffling diatribe.
The reason people routinely employ a safety device like a helmet or a seatbelt is to mitigate the risk of something bad happening. Risk is a tricky concept. It's a mixture of likelihood -- "how likely is it that something bad will happen?" -- and consequence: "if it does happen, what's the outcome?"
So, people who live in a city and regularly walk to the shops often wear shoes and clothes but don't routinely employ an armed entourage and body armour. Why? Because common sense tells them that it is very unlikely (even in Hackney) to get shot on the way to the shops. Going without shoes, though, would very likely expose them to cold and dirty feet and a feeling that they look like a complete tool (even in Hackney). So for urban shopping, no special safety measures are, rightly, the norm.
Let's try the other extreme. F1 Drivers wear alot of safety apparatus including full face helmets and neck braces. Why? Because experience has shown that many racing drivers who don't utilise such devices will die. In F1, the likelihood of a crash is high; the consequence is high.
So how does this apply to our own little cycling world? Here are three random but representative extremes.
1. Let's say you routinely commute by bike for 50 minutes each way on a busy 'A' road, in all weathers throughout the year (like the Curly-haired lady on the A1079 does). Common sense tells us that the likelihood of an incident is quite high (car, bus and lorry drivers are not all attentive angels, and there are many of them passing every cyclist each minute); and, given the speed of the traffic and the vulnerability of the human frame in contact with moving metal, it's safe to say that the consequences of an 'off' would be pretty damn high too.
In such a situation I'd say that Curly is mad to even attempt what she does. Wearing a helmet, high vis. gear and sticking a little red lollipop to her rear carrier merely seems like decorating the deckchairs of the Titanic with a smear of, er, icing. Curly-haired lady is one of my heroes, but she's taunting the risk gods. To bring her personal risk down to acceptable levels, what she needs is a strong, weatherproof exoskeleton fitted with speed and impact retardation devices: yep, a car.
2. On the other hand,
a pal of mine has been living, and riding off road, in the Peaks for many years. He likes the challenge of rock strewn, water-gouged descents, heather flats and very narrow twisty 'C' roads. BITD he had an off and spent time in hospital with a broken face and sore head. Ever since, he has worn a helmet while riding. He's even fallen off a few more times since, and says his helmet has stopped his head from hitting some rocks.
For him, the likelihood is high enough to worry about and the consequence, pretty troublesome: experience has shown him that wearing a bonce trap will stop inconvenient contact with the NHS. He's made the right decision.
3. What about me? A creature of habit, I ride solo on grassy chalk downland. High likelihood events include getting sheep shit in the eye, cattle flies in the eye, mole hill mud in the eye. I usually wear glasses while riding: I had a pair of clear MTB ones but my Favorite Pointer Bitch ate them. So I went for a ride without any glasses and got a fly in my eye going fast down a 1 in 4 grass slope. I had to stop pretty quickly (well, canti quickly) and rub my eye before continuing on my way. Low consequence. I must get some new glasses. And a crud catcher? No - that would make me look like a tool. I'll put up with the extra likelihood.
When deciding whether or not to wear a helmet, each of us has to assess the risk and make a decision based on experience. If you have no experience, err on the side of caution.