I'd forgotten about Vislor's Leech. Maybe this idea could be moved to that discussion.
I wouldn't underestimate coin tokens. An unbounded discarder can pin you, but this can't. If you buy my good cards, I'll still have coin tokens to buy things on my turn, with or without cards.
I suppose the danger then is a combo pin with Possession, but that's not easy to set up. You need a way to play multiples of this card and Possession as well, and to play it consistently would be difficult as well.
If it is really a problem, it could be bounded in the same way that Pillage is.
I think you are underestimating the trashing and overestimating the coins. This can pin you just as well as an unbounded discarder. Afterall, if you use that turn to buy back what you lost, it's the same as if you got that card discarded AND lost a buy.
Imagine you play this 5 times in a turn*. The other player has zero cards in hand, and tons of coins. Neat, what I am going to buy with all this money, she thinks. There's two possibilities: either she buys a treasure or action card, or a victory card:
- In the first case, it doesn't really matter what she buys, because she won't get to play it: you will trash it before she does.
- In the second case, there is the danger that she will get half the points, but you know, if she is getting all her useful card trashed and is greening aggressively in response, you will simply stop trashing her cards, because she isn't going to do much with a deck that is half coppers and half green in turn 12. Or even better, you will simply buy her provinces after a while. Sure, she can buy a province right after, but her total victory points aren't increasing, while yours are.
I think the pillage clause is necessary here.
EDIT: it is absolutely not necessary to play this 5 times to pin the other player, I'm just taking the extreme case.