I use the thief sometimes, but just as a BB replacement for flavor, really. They aren't as good, but once they promote they do the job well enough.

Black Mages are usually part of my party, but I do think they are on the weak side in the original game, as you are pretty heavily MP gated and Red Mages offer comparable potency with the benefits of white magic and a salvageable physical attack. I find they make grinding pedes and trolls a lot easier around Crescent, but then I tend to move on to the Ice Cavern for Eye grinding, which kind of backseats them again.