I'm trying to think of a way knives could be useful for there class but not break the class at the same time, for example maybe they could have a decent crit value but not overly high and 1-2 range, so your thief can attack without having to worry about getting countered all the time and they can inflict critical hits every so often. This can actually balance quite well because if a thief went up against say a knight or something with decent defence then the knives wouldn't hit well at all or just no damage at all and the critical hits wouldn't make much or any difference.
Sure it isn't full proof but I've only just thought of it, it seems like the physical alternative to a mage.