I didn't get my thief playthrough very far, but from what I've seen: Eloquence is necessary to solve some quests but I don't think it's worth it past Apprentice. Agility is good if they actually get to you. If you're playing a thief, you're probably going ranged; still, this skill helps with survival a lot. Ambush is wonky as hell when deciding if it actually wants to trigger or not. I wouldn't waste points in there. Stealth is weird. Sure, it's what makes you a thief, but most of the combat areas are just way too crowded. Off the top of my head, I can think of around three places where it could come in useful in the entire game. Stealing from shops is worthless and has way too big a failure rate. Money isn't really an issue in this game. Thieving/opening locks is great - except there's a low-level spell that easily opens most of the locks you'll encounter until almost the endgame. Magical attack actually gets pretty powerful later on.