I never had a helmet when I was a kid, because I couldn't afford one. I was pretty **** lucky that, despite falling off a lot, I never got seriously injured.
When I got back into riding, you know what I did? First thing I did was go buy a helmet. Okay, so it's not a million-vent million-dollar helment, but it's comfortable (it took us ages to find the right size and fit in the store). Yeah, in the summer it gets horrible and sticky, but so what? I've fallen off a horse recently where my head was about three inches from being skewered on a jump stand. If I hadn't been wearing a helmet and collided into that stand, I wouldn't be here (though, honestly, if I had hit the stand and been wearing a helmet I still would have had a major injury).
And my solution for the summer stickies? Shave your hair off. I had hair down to my ass that I loved, and I did it. Got lots of people involved and donated to a cancer thing too. But quite honestly, if you have the money to afford a more expensive helmet than mine with extra vents and crap, you shouldn't be having a big summer stickies problem. I had to compromise with mine because the only good fit and size we could get in the store was a Dublin Onyx. Also, get black helmets. White ones go yellow quickly, and even in a hot climate the colour doesn't make much difference with heat attraction.
I've also trekked a lot, and I never once refused a helmet. Once I had to wear none because they didn't have a helmet that fit me properly (and wearing a bad fitting helmet is worse than wearing none because it can impair your sight by falling down or simply give you a headache and cause a lack of concentration). But I'll never not wear a helmet purely by choice, and I'm just shocked it's taking big falls to make people to realise they should be wearing them. I've stopped my younger relatives from watching competitions before (mainly Dressage, I don't bloody care if a Tophat is traditional, it ain't going to protect your skull if that pumped up stallion of yours decides to use your head as a starting block) where the riders aren't wearing helmets. I don't want them to have bad role models and think they can get away with not wearing a helmet (which they don't when they come to my place to ride).