Maybe you just need a copper from the ambassador so you can trash two cards with your mercenary...
What? Ambassador doesn't put cards into your hand, and there's not a Reaction that will do that either.
Ambassador doesn't, but Smithy and a lot of other cards that you can play on your own turn do.
I suppose, but we're talking about the very late game here. Your deck is likely already full of green cards, so it's not likely that you'd draw the gained Copper even if you gained it. Maybe you have a Champion in play, so that you can first Scavenge a few Coppers, then draw them with your Moat (since we've already granted that you have one in hand), then trash them with Mercenary.
But in addition to it being the end-game, it requires that you have a Champion in play, plus a hand consisting of Scavenger, Moat, Mercenary, one junk card, and one non-junk card (or without a Champion, the 5th card can be a Crossroads). This is an impressively edgy edge-case.