I believe that a player should be allowed to concede, if it doesn't affect the game, or if it affects the game in a POSITIVE way.
In a two-player game, that's easy. Conceding shortens the game without changing the result, and is an act of good sportsmanship.
In a multiplayer game, it gets trickier. That's because the conceding player may have assets that would affect the balance of power between the REST of the players.
In Monopoly, it could be that there is a "stalemate" between two players, with a third clearly behind. Then what happens to his properties if he resigns? Do they go back to the bank? Does this open up the game for BOTH remaining players, or is one more likely to benefit than the other? If BOTH remaining players agree, then the third player will do well to resign. Maybe the resigner should distribute his properties in an equitable way before resigning.
But it might be that one player is benefiting from the stalemate, and will likely lose if the third player leaves. Then it's essential that he remains in the game. (Maybe he can leave physically, and let some other player "play" his game for him.)
In Diplomacy, a player resigns by declaring civil disorder. But his pieces remain in place, and other players can make alliances with the individual cities to defend them against third parties. So the resignation is "easy."
In Settlers of Catan, a person should be allowed to "resign in place" as in Diplomacy. That is, his cities and cards remain with him (and out of play), but he doesn't continue playing. Other players can use a Monopoly card or the robber to deprive "him" (his position, actually) of some of those cards. This deprives other plays of the chance to trade with him (a minus), but also puts him in the position of NOT having to play "kingmaker" (a plus).