I'm sure everyone agrees that Thief class is in need of re-balancing. The Thief is generally inferior at combat (particularly in BG2, when missile combat isn't that useful). The one case where a Thief is useful is when it performs a backstab, which does a massive amount of damage. But backstab requires a lot of planning (potions plus skills) making it not the best long-term option. I contend that for many reasons, Sneak Attack is superior to backstab, and should be implemented instead.
Sneak attack is easier to do (requires only flanking) and applies in more circumstances. It therefore can help rescue the Thief's poor combat performance. It's also a more balanced form of combat than backstab, since sneak adds only a few additional attack dice at most (rather than multiplying base damage by some ridiculous factor, like x5). And since a rogue never gets that many attacks, even when magically enhanced by items or potions, Sneak attack never gets that overpowered.
Sneak attack still requires positioning, so you'd have to micro-manage your thief effectively. This, if anything, adds to the combat experience. You could apply it to any weapon, which removes that absurd "swords only!" requirement of backstab which limits variety. And, if you were really worried about balance, you could make it apply once per round only.
I know Backstab is a favourite of 2nd edition diehards. But it's vanished from the gaming scene since, and I think there is a reason for that. In 3rd edition and beyond, Sneak attack was a huge part of the Rogue class. The Sneak attack mechanic has also bled into other games, like Dragon Age. In Icewind Dale, another Infinity Engine game, they tried to implement this system and I think the results were good. Sneak attack is clearly the wave of the future and the salvation of the rogue.