It's flexibility and hybridness, but since he was too damned good at it, and his way of doing it never built towards anything "better" (in the same vein), he was cursed with awesome, and failed by winning
When you translate his upsides into concrete results what you get is more powerups (which makes you "naturaly" outperform any other equal level class by a small margin), more gold and shops (which translates into more CP and that then can turn into all sorts of stuff, including piety for converting), 1 more glyph (?) (even more flexibility and CP), more potions (which also do double duty, so even more flexibility)... Stabber makes you commit to spikes, and the potion antics let you spike with both phys and mag at the same time.
When I think about it, he teaches you about either cheezing or scumming for stuff
Apart from that, he's the go-to Conversion Points guy, and mastering him builds towards being good with the Tinker rather than anything else from the Thief Den. The "buy and convert" cheap item shennanigans are also his domain as far as early kingdom develoment is concerned, and more leeway with gold let's you experiment with stuff other than the Sword or whatever is a must buy ATM.
AMUSING BIT OF TRIVIA: He was the go-to guy for the 3rd or 4th class completion slot for "complete with X classes" quests, especially Vicious ones. It was a bit absurd once you got into a habit, you'd use whatever class was a natural fit for whatever dungeon, you'd go either Monk + Paladin or Rogue + Warlord (depending on the player / dungeon), and then you'd look at the pool, shrug, go in with the Thief and win without being able to pinpoint exactly why it worked.
Once I figured that out, I started doing the Thief first
I wouldn't learn a thing from it, but hey, the number of times I walked in somewhere with a half-hearted thief and walked out victorious was mind boggling.