Mangum has not shown that he is anything special as far as a BYU QB goes. However, for the most part, he gets the job done in Grimes' offense. To me he is a collegiate Trent Dilfer. The problem is that while Dilfer was asked to manage the game, he could stretch the defense when he needed to as part of managing that game - Mangum and this group of receivers cannot do so on a regular basis.
Yes, there have been some big drops, but in years past, I remember plenty of drops with other QBs as well. The idea that receivers don't drop passes is simply incorrect; you see guys in the NFL dropping uncontested passes every Sunday. This is where the QB needs to be that guy that scares the defense and makes them think that they better cover that short, slow receiver who runs an average route because this QB is the type of guy who will figure out how to get it to him if he is at all open. While, yes, our receivers don't seem to be making plays or fighting for the ball, Mangum has not shown the ability to keep the defense honest either. They are not respecting him, and I am guessing that is coming from opposing coaches having made the decision that Mangum is not an every down threat and projecting that in practice.
This is why, as I believe Edwards famously quipped, the backup quarterback is always the most popular guy in Provo. Because when BYU has special years, they have a QB that silences the part of the BYU fanbase clamoring for the backup by scaring defenses, keeping them guessing and figuring out how to make a play - regardless of who is on the field. For me, the touchdown passes from Detmer to Bellini and Salido versus Miami typify the type of gamesmanship and playmaking that a BYU QB needs to have for a special season. Hopefully, I'm proven wrong this season!