After seeing the news about a recent skiing accident in Germany (Two people collided with each other and afterwards with a snow cannon, both dead) I was wondering how much extra protection a back protector offers. In the report it was noted that both were wearing helmets, and none of the helmets was damaged. Therefore: How much does a back protector/vest add in additional safety? When is it worth, and when useless?
I spent a lot of time in the mountains of Austria in years past, snowboarding. In my experience I've seen a lot more snowboarders with serious back injuries than skiers. That may be because of the greater potential for catching a backside edge and going down back-first, but may also be because I surrounded myself with more snowboarders than skiers (maybe a selection bias issue).
I wore one of the plastic cockroach style back protectors for a while but found that it served little more than to limit confidence in my mobility, which, on occasion, got me in more trouble than I would have been otherwise. I am a staunch supporter of wearing a helmet (ALWAYS wear a helmet), but unless you are just getting into park riding or big back-country riding, I would skip the back protector.
There are certainly studied merits to the effectiveness of back protectors, and I would recommend them to young, developing riders in certain instances, but accidents like the one you address are such freak accidents that the outcome would likely be unchanged with an additional piece of gear.
What I DO wear however, are my old lacrosse rib pads as I've found my ribs to be of greater concern than my back and have suggested those to friends on more occasions than a back protector.