There's a lot of talk in Competitive TF2 about the speed of play. They don't want multiple medics because it would slow the game down. They don't want multiple heavies because it would slow the game down.
You know what slows the game down? 5 point maps. They don't punish waiting, since neither team is losing anything while time goes on. In Payload and Attack/Defend maps, one team is always on the clock, so there's always an incentive for action.
Payload and Attack/Defend also make the less used classes more legitimate as well.