We all snipe occasionally. Accepted and forgotten.

Well, I did say "possibly", and have said in the past "at best", and I do believe there are scenarios that this can be life-saving, but fair enough; it was a touch of hyperbole.

Studies have been posted about risk compensation, and I stand by that this does occur to an extent, but am willing to admit I don't know how much.

In that extreme, likely no, I wouldn't expect people to know; I'd also note I'm not sure that people are consciously making the choice to engage in riskier behavior "knowing" that a helmet will save their life. I think what most of us advancing this point are saying (and, I believe, what most studies point to) is that people subconsciously factor this in to their decision making. As I said, whether or not you know it, you are probably more likely to engage in riskier behavior if you're wearing a device that is supposed to save your life in a catastrophic accident. Closetbiker has posted some of these studies in the past. I agree they aren't concrete in all cases, but it's a starting point, and surely enough to maybe make us stop and think if perhaps there's something to it.

This has been asserted in cars as well; with the advent of airbags, seatbelts, etc, people are more willing to drive faster and more dangerously (probably not purposefully), since they perceive it as more "safe". The risk vs reward assessment changes; the risk (death) is seemingly reduced, while keeping the same reward. That skews the decision. We make these decisions every day so often that we are mostly unaware when we make them. From everything to crossing the street, deciding whether you can make that yellow light, etc; you're always making these choices, whether or not you know it. Constantly and subconsciously assessing our situation is a good piece of evolution that has helped us survive this long.

EDIT: There was actually an article in the recent National Geographic: How Teens' Brains Work article that is somewhat applicable, and I'll see if I can dig it up (or at least post the relevant text). The gist was that the reason teens act recklessly is because their risk/reward assessment is different at that age, in this case the opposite of what we're talking about; it's not that teens don't gauge risk correctly, in fact they do so quite well. The problem is they put greater value on reward than adults do, which skews their decision and makes them more likely to engage in riskier behavior, completely subconsciously (they go on to explain that this is an evolutionary feature, not a bug, and why, but that's beyond the scope of this thread and not relevant to the point at hand). Can you see how this might apply to other skews in that decision-making process?

I don't know, but I don't think it's very improbable that a lot of them are under the prevailing impression that helmets save lives, and so decided to wear one thinking it will do so (at the very least sometimes). Perhaps some of them are aware they don't and wear them anyway (I would, in that case), perhaps some would decide not to do it at all, perhaps some would be a little more conservative. Who's to say? The problem is, we don't know if they don't know.

I think they are aware of those limitations, yes; my experience has shown me most people are not aware of their limitations in protecting the head. Not that I blame them, I wasn't either.