I think there's also something to bear in mind is the context of Thief: in base set alone thief has different potential than when all set are combined, and as mentioned thief varies with the numbers of players too. I've had games grind to a halt because of thief due to there not being enough +$ (in fact in that game the only +$ was mountebank which just made the situation even worse!!) and I've certainly had to think twice about going chapel when Thief is on the board, especially in 3/4 player games which is what I normally play. In 2 player, yeah, I may well ignore it, but I mainly play 3/4.
People seem to be so keen on labelling a card (and stuff in general) as "good" or "bad" so that they can pigeon-hole it, which means they don't have to think about it any more, the decision is made. This is naive and also leads to irritatingly polarised opinions such as "it's so obvious after a couple of plays that Thief is a trap card" .....really?.....REALLY? Maybe given some particular circumstances and events perhaps, but ........ no.
Re: Why is Curse a base card?
Because you've misunderstood themes in this game. Themes are layer on top of the basic mechanics. The basic game requires a certain amount to basic mechanics to be varied and fun - you cannot split absolutely every mechanic off into it's own expansion as that would be incredibly dull for the first few of sets until you got enough of a mix to make it interesting. Cursing is included in the basic mechanics as are trashers and dead drawers as previously mentioned as core elements of the game, not optional flavours of the game.
And also, yeah, you're only going to get X amount of cards in total - whether those curses appear in the base set or expansions it's still the same amount of cards.