[Nerull]

As someone who mostly plays "all-good" parties, I can truly say I use Imoen as my thief. The main things you need to a thief for is to open locks and disarm traps; the rest is just bonus material (though some would argue that backstabbing is vital, I have rarely used it in the past, and have finished the game just fine). If you really like backstabbing, consider adding Valygar. While he does not have as large a multiplier on his backstab as a true thief of higher level, he does use a fighter's THACO, allowing him to hit more often than a thief usually does. You are right about Nalia; she is worthless as a thief (without potions, at least).

Imoen will be just fine for your thieving needs, especially if you ride your reputation up to 20 like I usually do (Jan will stay in the party, but you'll have to listen to him gripe the whole time). If you want a thief that you can do everything with (pick pockets, set traps, etc.), you are pretty much stuck with Jan unless you play a thief yourself.