True story - the very first time I wore a helmet in the BC was a few years ago.  It was early season with a typical thin, faceted rockies snow pack.  Last run of the day I core shotted off a rock and became a 200 lb star fish cartwheeling head first into a band of raggedy limestone. Later, back at the hut we all marveled at my helmet which was completely crushed and shattered on one side with small pieces of rock embedded in it and chunks of black plastic shell missing.  I shudder to think about it now, but I don't think the fall would've been survivable had I not been wearing it.  Why I put it on that day and no other (non-resort) day previously boggles the mind a bit - but thankfully I still have a mind to boggle!

Anyway, I think the biggest reason why backcountry skiers rarely carry a helmet is that you really only need it for about 1/6th of the day (i.e. the "down") and the rest of the time it's either taking up too much space in your pack, or hanging off a clip and banging into your ass while filling up with snow.  Alternatively, you can actually wear it on the skin track and look like a total gapper while sweating like a pig - don't forget to put your goggles on to get the full sauna effect.  

The solution for us came when my wife and I upgraded our ski packs to the new Mamut Nirvana Pro ski packs which have this handy built-in helmet "hammock" that keeps your brain bucket secure on the outside of the pack.  Now there wasn't really an excuse for leaving it behind and helmets have since become "standard" equipment along with beacons, etc.  Luckily, neither of us have had any more rock vs. head episodes but I've got a few good scuffs and dents from various tree skiing episodes.  

...now, another interesting question is whether having a helmet on makes you more reckless!?  

That's always the way! very fortunate.

With the new packs, carrying a helmet is a synch (other than the added weight) and with the added dangers in the BC it only makes sense to wear one, but the vast majority of people I ski with inbounds all wear helmets - then as soon as they go OB the helmet is no where to be seen. Kind of strange but like you said it is only used 1/6th the time.

I had one for 2 years and skied far more recklessly and had a few close calls in bounds in the trees, got rid of the helmet and no more close calls. False sense of security if you ask me! They do Zero for neck injuries which is the vast majority or sever injuries minus of course concussions which are a big deal.

The debate goes on, but from here on out I am a toque guy, always have been always will.

(unless of course they make them mandatory).

That's interesting though, because I'm sure we've all heard similar logic applied to avy beacons, life jackets, seat belts, etc., "if I bring it I'll take chances I wouldn't otherwise take".  Maybe that's human nature?

The other thing is that there are probably only certain types of impacts and range of speed that a helmet is really going to offer protection for, i.e. slow to "moderate" speed and impact with sharp or potentially penetrating or abrading objects (as in my case above).  If you're doing 65 and slam into a rock the helmet is not going to stop your brain from sloshing around in your skull and probably killing you anyway.  If you tag a tree doing 25 and a pointy branch nails you on the forehead, I'm thinking a helmet will help.  

Neck injury is probably going to be a factor with or without a helmet but I'd be interested to see if helmets increase the risk somehow.  If they do then it's a bad design flaw!

Truth be told, I agree and would rather be wearing a toque because it just feels right.  On the other hand, I like my brain and figure a helmet is going to offer some safety advantages in at least some instances.  Meanwhile, there are still no substitutes for personal responsibility, common sense and skill.  ...all things I hope to demonstrate some day!