The Bite-Sized series of articles is meant to take the most powerful cards in Dominion and give a short explanation of what makes them powerful.
Junking attacks in general are pretty powerful in Dominion, but Mountebank is able to potentially give out two cards worth of junk on each play, making it one of the most powerful junking attacks in the game. Even after the Curses are gone, Mountebank can still junk opponents by giving them extra Coppers.
Decks with lots of Curses can dodge Mountebank’s attack, however, so after the first few plays, Mountebank’s attack can get less effective. To mitigate this, you may want to switch to another junker later on if it’s available, or try to play multiple Mountebanks in a turn if possible.
If your opponent has Mountebanks and you are faced with the choice of trashing a Copper or a Curse (which you may have a few copies of), consider keeping the Curse around to guard against future Mountebank attacks. Often times this can be better for your deck control, and this will outweigh the payload you get from the Copper, especially if you can trash the Curse later.