I think the concept is cool in 2-player games, but I can imagine it being really frustrating with 3+ players. Al rushes Minions, so Bob gets Commonwealths, and now you're stuck without any Minions, and they crush you because Al's got an awesome engine going, and Bob's got all these points from Commonwealth, and if you try to compete for Minions Al will just pick up some Commonwealths of his own to make matters worse; and Commonwealths won't be worth many points for you (except maybe they will because Bob is getting lots of Commonwealths, but if you extend the example to 4-player then you could definitely get stuck).
Also, even if it trashes I agree it should exclude Copper and Curse. It would be awful to get hit hard by Mountebank, then just have it rubbed in as your opponent picks up Commonwealths to score a billion points off your Coppers. I guess if it trashed a lot of cards you could make up for it, but I think it's a lot more elegant to just leave it as is, and then it's function is just a pure victory card.