As with every other position, you're always one hit away from having your number called. You look at USC, they're on QB3 right now (heck of a job by Helton for holding it in the road). At least 80% of the posters here have either complained about or expressed concern about our offensive line's pass pro problems. Add that to the fact that kids can (and have) bailed in the middle of the season to other schools. You can't get caught off-guard.
And to add to that, the defense isn't the only place where we need to get freshmen all the reps as possible. With next season's schedule, you want to be breaking in as few players as possible. And it's very possible that Lia will win the spot over Mac in the spring. You don't want his first live bullets to be against USC.
So this has more to do with coaching and looking ahead than it does with QB play.