well, the idea is that making embargo the bane DOES make me more likely to buy embargo, but if moat is the bane it doesn't make me more likely to buy moat. I mean, in a game with embargo as bane and no moat in the kingdom, young witch is stronger than in a kingdom with moat as bane and no embargo. However, when comparing cards as bane, I'm thinking of comparing the difference between card X as bane and card X as not-bane to the difference between card Y as bane and card Y as not-bane
good point about being able to block after they discard, though that usually doesn't matter.
to explain:
consider a kingdom with young witch and every single other 2 and 3 cost in the game of dominion. The bane that makes young witch the strongest is obviously moat.
Leaving aside that YW is completely ignorable in a kingdom with Scheme, Hamlet, Village, Forager and Watchtower, I don't see why this is true at all. If there's even just Village and some cheap source of +buy (Hamlet, CM, WV, Forager), I don't mind getting quite a few Moats, which would make YW pretty bad for you except that it has forced me to get some Moats over potentially available better drawers.
Embargo is clearly so much worse as a bane because you just don't want to keep it in your deck at all.
Well, the idea is that you can get a few moats no matter what bane is, with the same effect. If embargo is bane, you have the option of getting that instead of moat to block young witch, in the very rare scenario where that is a good idea.
and yes, YW is bad in that kingdom, but that's not the point.