In a KC-Mountebank scenario, getting the deck size down before getting hit is really important. You risk getting snowballed really hard, as you cannot trash effectively and also cannot connect your own KCs with the important stuff once you are getting 6 junk cards a turn, and if the opponent manages to connect KC-KC-MB-MB-MB is probably game over. Other heavy Attack things like KC-Saboteur also like the slim deck quick, as the trashing Attacks cannot target cards in hand.
But the most common is likely Goons+Village+no draw, in which you need to get the deck size down before starting to get hit by Goons, otherwise the loss of tempo is huge, possibly worse than the loss of tempo of buying a second Chapel.
