[Lifetime]

You need a Fighter/Thief to be able to reliably pull off Backstabs at the lower levels, otherwise your thief is in a really really bad way.
Sneak attacks tend to be less effective, since you can only sneak attack every enemy only once in a combat and the negative effects of the sneak attack dont last very long. And in IWD its not really trouble to hide your thief and backstab, but it IS difficult to get past a whole bunch of monsters to attack at their flanks. Moving around is always easier when you're not in combat, so sneak attack issint as useful as it sounds.