Even without reshuffle considerations, if you have Mountebank and 3 Coppers, TR is enough to buy the last Province and win, while KC may give the opponent the extra cards to pump up his 8 Gardens and make you lose the game, Province or no Province. There are also scenarios in which you would like to play Beggar twice but not three times (you want $6 now, but not the extra unnecessary Copper), scenarios in which you want to play any forced trasher a specific number of times (Trade Route or Forager for cash would not even be that edgy).
Some other Attacks that junk can deplete piles if you play them more than necessary. Also, stacking too many plays of any gainer may be bad for your deck and/or your pile status.