I'm a reductionist. The derp of backstabbing/sneak-attack isn't how rogues do it, but that they do it at all. Clearly it's a martial concept which should be solely in the warrior's repetoire. Backstabbing is a holdover from OD&D days when every character was assumed to be 'good' and honorable tactics were the norm. You're trying to tell me there's some aspect of physical combat that not only are rogues better at than warriors, but that it's actually forbidden to warriors? Give me a fucking break. It's just another step in the decline of RPGs, albeit an early and now-accepted one.
'Backstab' should be what every character does when attacking enemy that can't defend themselves from this attack (which is the norm when the enemy is facing away from you).
Rogues shouldn't backstab a lot because it's some awesome ability exclusive to those following the shadowed path of roguery, they should backstab a lot because:
a) stealth and subterfuge allow them to put themselves in position where they can attack someone without them seeing it coming
b) It's pretty much the only way for them to do something of use during actual combat.
Every other class (including mages) should backstab too, they should just get less opportunities to do so (debatable with mages who can cast stuff that paralyses, immobilizes or makes the target fall asleep).