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.
The issue I see is that Rogues will be Backstabbing most of the time, and if those hits can't benefit from Crit Chance then anything that increases crit chance will have very low appeal (Why put points into a stat that only helps in a situation that rarely occurs?).
As a suggestion, perhaps crit chance could increase critical damage in the case of a Backstab. I mostly don't like that half of the stat value of Wits is wasted.
I don't feel like the balancing would be hard at all.
Crit Chance can be given out at a balanced rate for its own DPS purposes. I'd say it already is, aside from that fact that everyone gets it for free with Rage. Also, Backstab is already balanced around Daggers having a lower base damage than other weapons. Making other sources of Crit do nothing is just extra punishment.
If Backstabs can't crit it will just force players to pump alternative DPS increasing stats, avoid gear with Wits in favor of Finesse, use alternate skills, etc. In other words, it'll be a pain and reduce viable playstyles, but likely won't reduce overall power too much.
And as far as the thematic element, I doubt all stab wounds to the back are of equal severity. I'd wager a person could live through many, and others would be... critical.
