I buy the very best helmets money can buy, for these reasons alone.
Yes it doesn't make me invulnerable, but I sure as hell would prefer to hit my head with a helmet on, than without one on... AND on top of that, being one that's been tested to death by a reputable company, conforming to multiple security certificates.
I help run a local MTB spot, and if we find anyone without a lid on, we eject them, simple as that. We've had complete beginners wanting to come have a look around and pootle along the tracks, to guys who have just dropped a lot of cash on new expensive bikes, without lids (for some reason these tend to be the Eastern European lot). We treat them all the same. Have had one get quite aggressive with me about it, but still ended up forcing him out with the help from backup / numbers behind me.
On the note of Boardman riding without a lid on, that's the stupidest reasoning I ever heard for not riding with a helmet on.
The reason given, was that statistics show, it's in fact better for general health to not wear a lid, than it is to wear a lid. What they failed to mention, is that the research in question was comparing the relative merits of more people riding bikes on the road = more active population = healthier population, VS the small minority who actually fall on their head and hurt themselves. Boardman has a vested interest in selling volumes of bikes through high street retailers, so obviously he's an advocate of "not wearing a lid".
I'm severely paraphrasing above, but that's the gist of it. I understand that with road riding, you're far less likely to hit your head than in MTB... but it really is my opinion that you're a complete moron by not protecting your head in the off-chance of hitting the ground with your noggin.
I personally would have been dead, without a shadow of a doubt, in some of the crashes I had... had I not been wearing a helmet.