Scott, I agree with you. I do long ski tours, and by the time I carry a first aid kit, a repair kit, a snow kit, a shovel, probe, saw, down jacket, waterproof etc., my pack is heavy and stuffed. Sure, you would be slightly safer wearing a helmet for every down hill run, but the truth is backcountry skiers are more likely to be killed by avalanches than head injuries, and traumatic deaths from avalanches aren't generally of the kind that a helmet is going to save your life. Plus, when you are skiing far from the road, you need to ski in control. It's like alpine climbing, taking a fall off a solid bolt at the sport crag isn't as big a deal as taking a fall off a manky piece of gear two days walk from the road. Of course, if your idea of a big ski day is a half hour skin up from the road, that helmet isn't such a big deal.
But, I've gotta admit the people I see with helmets in the backcountry have zero other safety gear so should they break a binding, get benighted or get caught in an avalanche, all of which is much more likely, they'll be hooped.