Are usually quite useful for finding hidden items. Perhaps some sort of detect hidden passages and containers and stuff like that... Basically to the point that if you go to hell and you want to fully explore it, you bring a thief, because he'll spot the things other people would miss. Of course this would be a lot of work, but just think of all the stuff you could find that would make having a thief around worthwhile for all sorts of exploring (and even when ranking... on the off chance they decide to share that they just spotted something neat).