I can only imagine the volcanic eruption if Penn State, behind whichever QB is playing, is down in Auburn, up by 14 points and in good shape in the game, and Franklin makes a change in QB- to make sure the other kids get meaningful snaps, a few bad things happen, momentum is lost, and Penn State ends up losing the game.
By the way, that philosophy is what is keeping Michigan from being 12-0 right now. Fortunately, for them, they are likely in the CFP anyway - but that could just have easily, in another year, kept them out.
To paraphrase Herm Edwards - "You play the QB to win the game". And if the Head Coach doesn't know which QB gives the team the best chance to succeed, you have an incompetent head coach.
When a QB is struggling, and you have someone on the bench who you feel could jump start things? Sure, you make a move.
When you are up more than 3 scores after halftime, sure, you clear off the depth charts - but how many quarters of play, out of a full season, is Penn State likely to be in that situation? Last year that would have been three possessions each against Ball State and Villanova, and one possession against Rutgers (and of those 7 possessions, I think the back-up was in at QB for at least five or six of them, include one for Mason Stahl)