Oh man, Talisman??? I'd better load up on Thieves so I can steal those! God, I just can't wait to steal some Ill-Gotten Gains too!
A big list of cards that Thief can steal is a lot of mostly-meaningless sound and fury. Noble Brigand is going to be much stronger than Thief on average at winning games. If you don't think so, hey, let's you and I play a bunch of Thief boards so I can pump up my rating.
There's room for both cards, sure, but one is much better than the other on average.
Ooh, lets pick on the worst treasures. If you don't want the Talisman, trash it. You can just trash it with the Thief unlike the Noble Brigand. Assuming you're playing against semi-competent players, they would not have a Talisman unless they had some strategy for it in which case trashing it on them early serves a purpose and you do not have to gain it.
Stealing an Ill-Gotten Gains with a Thief a bad idea? I'll happily
gain the Ill-Gotten Gains from the trash pile so I can hand out the curses. As noted above, I'm assuming competent opponents so there's likely a Upgrade variant of sorts in the kingdom and not any better curse dealers available.
I won't forget the Harem/Fools Gold game I played. You should have seen their faces when I pulled a Thief out of the Black Market deck. If it was Noble Brigand I would not have have bought it.
Oh, and I'm sure you will be able to point to isotropic and show that Noble Brigand has a higher win rate than Thief (in games that are 90% only 2-players). I'm also sure that in general, playing blind without considering any strategy of either player, Noble Brigand will play better. But actually playing with strategy is different. You NEVER buy a Thief without a reason, so it will always have lower stats from players who have not learned this.
But step away from the screen and play in a real game. If you want to play with stacked Thief/Noble Brigand kingdoms, come over to my house and play some 4-6player dominion with the group I play with. Here's how it goes; Thief. Thief. Thief. Thief. Thief. Your turn! Buy that Noble Brigand and you'll see every Silver/Gold get trashed instead of gained.
Donald X is no fool. Cards have been play tested for a long, long time.