When folks criticize an rpg system it sometimes happens that a fan of the game will reply "That would only be trouble if you're playing with munchkins." There are two implications at work here that everyone needs to understand. First, the guy who brought up the munchkin defense is implying that the critic is or plays with some sort of abusive player. The defender of the system might as well be saying "Oh, my group NEVER has that problem, because we're such superior players. Unlike you and your posse." Blaming mechanical flaws on the people playing the game won't make the game better and it sure as hell won't make you any friends.
But there's a deeper issue at work here as well. The designer very often should be catching some heck for rules that are easily abused. In the real world there is often little difference between a hardcore munchkin and a player who intelligently maximizes his interaction with the rules for optimal performance. The primary thing that distinguishes the two is that the latter understands that the game has a gentlemen's agreement in play, some sort of unstated code like "You the players won't bend the system until it breaks and I the DM will play fair." The munchkin either fails to recognize or doesn't understand that unstated code. And the usual reason for this disconnect is precisely because the gentlemen's agreement is all-too-often taken as a given. Don't leave the agreement unstated. Some people can't pick up on the subtle cues necessary to grok the existence of a gentlemen's agreement.