Is a "breakover feature" required for airliner seats?
I can't speak to the regs, but my understanding of the brace was primarily to protect your head, not being as exposed to any debris, and not having very far to travel to bang against something. 

As far as I understand (and I'm less sure of this), your back being in a curved position like this is better able to absorb impact/jarring than if it's vertical and you drop suddenly, or are whipped forward and backward by suddenly arresting horizontal motion.