In this case, Guatemala posed no threat to us whatsoever, so yeah the 13th player was unnecessary, but this is my reasoning:
Suppose you were playing in the first game a team like Brazil or USA with managers who most likely scout their opponents intensely as is to be expected from an NT manager. Since this is the first game of the season, by oeuftete's reasoning there should only be 12 players in the roster. Now these 12 players include 2 SF players that are known to be inside players, couldn't the manager of the other team be able to guess that you would play an inside offensive tactic and prepare accordingly.
Now, if you add a 13th player that is known to be an outside SF, wouldn't that give you strategic flexibility, not only in your roster, but make it not so obvious that you were going for the inside tactic.
I believe that even if you don't use a player in the game, it doesn't mean that it doesn't serve a strategic purpose.
Just my 2 cents.
Martin