A backstab is a crit. A crit that crits even more, sounds a bit ridiculous and also would be hard to balance. Backstabbing is a guaranteed crit, the normally crit will still be necessary for case, where you can't get into a backstabbing position.
Honestly I don't like the forced separation. It makes you much more dependant on good items because social points are totally scarce. I prefered to have the freedom to decide, if I want to put my points in my combat stuff or into my social stuff.
And as already somebody mentioned, leadership for example would be in reality more likely a combat skill, not a social skill. There is no 'social' benefit coming from leadership. And sneaking is at least part wise a combat skill as well, either for exploiting it in combat self or for using it to preset an ambush. Loremaster as well is partwise combat, because you need it to examine your enemies.
The more I think about, hardly any of those skill is really social. Only Persuasion and Bartering influence real socialising actions. Everything else is more utility based, I would say.
