The difference being we could have up to 5 midfielders midfielders on the pitch at any given time, depending on the formation we're playing. There will always be a chance for any of them to play any given game.
How many times does the number 2 LB get called upon. When would we ever send on an inferior LB late in the game, just for the sake of it? Only if Bertrand was injured or fatigued.