Not a professional thief by any means, but I did skill one up for SA dungeon to grab up some of the dungeon armies back then. Even though these have largely fallen out of common use in suits, they are still very valuable for turn in points!
I skilled my thief originally for pvp, as stinky Pete said it is a very unique playstyle, and not just within UO but within MMO gaming in general. How many games can you play where you can watch a well-coordinated group work together to accomplish a task, then treat yourself to their rewards? And IMO the risk of getting caught and ganked is the most exciting part.
Playing on ATL, though, I was getting ganked a lot more than I was getting away lol... And what's their reward for putting a stop to my nefarious plan? A chance to talk some sht, a stack of smoke bombs, and *their* item back. Haha
-edit- I would also like to add to the recommendation of adding ninjitsu. It's a very fun skill, with a lot of defense and utility. Cat form adds enough HP regen to replace the need for bandages. The mount forms allow you to 'walk' and remain in stealth as fast as an unmounted player runs. Unicorn form does this and also makes you immune to all of the lower levels of poison, which many ppl use to keep you out of stealth. Get the ferret talisman and get a bonus to stealing, if you're not already 120 (very good for dungeon stealing).
Ninjitsu has some very nice perks, even at lower skill levels (but I recommend Skilling it as high as you can). The biggest perk being smoke bombs, available at 50.0. Smoke bombs are a game changer, you will absolutely love them. It will certainly change the way you look at eggs...