In the same vein it's annoying to read that some cards were included for the sole purpose of looking strong but turning out to be weak (Thief)
I have never made a card for the sole purpose of looking strong but turning out to be weak. The cards are all there to provide gameplay; to be cards people enjoy playing with, that change the game in whatever ways they do. Thief does look strong but turn out to be weak; but wasn't the point of Thief. Weak cards get played less often; strong cards cause other cards to get played less often. The game is at its best with balanced cards. Whether or not they look strong or weak at first glance, that's not as important, although certainly they don't want to all look awful.
There are four kinds of attacks in Dominion, four kinds that matter anyway. There is an essay about this elsewhere on this site but I will list them for you: give other players junk (Witches); trash other players' good cards (I think of these as Knights); make other players discard (Militias); and muck with deck orders (Spies).
With only four kinds of attacks, I want to be able to do each kind in each expansion, to maximize the variety of attacks. Therefore I want to be able to do Witches in each expansion.
You can hand out stuff besides Curses. Estates are pretty bad. Copper isn't so hot. But handing out that kind of thing has issues. The Copper pile doesn't have a definite size, and varies with the number of players in a poor way. There are not very many Estates. You can do a few Witches that use cards like this, but most Witches cannot.
You can add new cards to hand out, like Ruins. The problem here is that that is a giant pile of cards to stick in an expansion. You would have to be making some kind of insane 500-card expansion in order to have space for those.
But by putting Curses in the main set, every expansion can have a Witch while devoting zero space to Curses.
