Well, it seems to me that "the girls" bailed Kivan out when he took a wrong turn and got himself turned to stone.
The reason I'd like to keep Jahiera is because she's a spell caster and spells are fun. A missile-lobbing spell caster is even more fun. On the other hand, there is so much stuff lying around that I don't bother with thieving very much. Currently, with Imoen dualled, I don't even have one thief and seem to be okay with missing the odd chest to lockpick. I do go through healing potions and spells when dealing with traps, though
But I can't see why I'd need two thieves.
Interestingly, I thought I'd be constantly on the lookout for better NPC's to round out my group. But now it's turning out that I am getting attached to them, for better or for worse. In terms of RV, it's interesting to have to work with the advantages AND limitations of your companions, just as you do in real life. Makes for more interesting gameplay, I think, than to create super-buff fighters that do their job effortlessly.