Tom Brady is the uncontested starter at the position. MVPs, All-Pros, Super Bowls, and one or two NFL records.
Brady has a 96.6 career QB rating, he is still in top form, playing the position better today than ten years ago.
Ryan Mallett has been the backup to Brady the last two seasons. Mallett played in four regular season games in 2012 during garbage time, where he sports a 5.2 quarterback rating. He has one completion in four attempts for a 25 percent rate, he has one INT, and 8 rushes for -9 yards (he was downing the ball to end the game).
He has a cannon for an arm, but lacks touch on many throws, and often lacks calm when in game situations.
Tom Brady and Ryan Mallett run a traditional drop-back (aka pocket) passer offense.
With Tim Tebow the Patriots switch to a more run-based offense known as the read-option (not to be confused with the Wild Cat which is an entirely different type of offense altogether).
Tebow has played in 35 regular season games, and sports a 75.3 QB rating. He has 173 completions on 361 attempts for a 47.9 percent completion rate. He also has 197 rushes for 989 yards, 28.2 Y/G, 5.0 YPA, 12 TDs and 14 fumbles.
I would expect Tebow's production per game, if thrust into a starting role, to look something like:
165 yards passing average, 1 TD to .5 INT per game.
50 yards rushing average, .5 TD per game.
50 percent completion rate on his passes
75 QB rating average
Although there may be some slight improvement, due to experience and a more favorable coaching situation. His production would be well below what is normally produced when Brady runs the offense, but one does not find many backups in the NFL capable of such.
Mallett does not have any real game stats to evaluate, however in the preseason game against the Eagles he was 9-of-18 for 97 yards, a 50 percent completion rate, not making great throws and missing some wide open receivers.
Mallett will likely depart via free agency after this season. The Patriots are likely to draft another QB to groom behind Brady, investing more time into Mallett past this season doesn’t seem the most feasible option for them, considering the cost there could be in re-signing Mallett to a new contract just to sit behind Brady for a couple more seasons.
They have Tebow under contract for two seasons, allowing them to see how he develops this season, and they can experiment and tailor their version of the read-option to his strengths and weaknesses. When they go into next preseason, they will be better able to determine what Tebow’s full potential is, see how far he has developed, and if it is worth investing more time into him.
If you practice something, and practice against something consistently, then it isn’t that big of a deal when facing an opponent using that style of offense -- familiarity takes away the ‘uniqueness’ of it. Tebow running the read-option on the scout team during practice, when preparing the team to face an opponent who uses such an offense, is one advantage to keeping Tebow on the roster.
Both backups bring something to the scout/practice field, and depending on who the opponent is that they are facing on any given week, one is far more valuable than the other -- but between the two of them, you have just about any type of QB/scenario covered.
Bill Belichick was asked on the Salk & Holley Show about the two different styles of offense:
"It's something that you have to look at with time management. The decisions that you make are important there; you don't want to waste a lot of time on something that doesn't benefit you," Belichick said on the program.
"At the same time, you want to try to be prepared for, and take advantage of, some of the players' skills that you have. I don't think it's uncommon. We've had those types of things in our offense before. This is a little bit different, but we're not trying to reinvent the game or anything. We're just trying to take advantage of a particular player's skill, and that's no different than something we would do with a tight end, or a receiver or running back who has a skill that we want to try to take advantage of."
"I'd say Tim is the first kind of running quarterback that we've really had here since [Matt] Cassel. As we know, Cassel gained a lot of yards in 2008 -- and he was a quarterback as a runner -- and we had a very good rushing offense that year. A big part of it was due to the  yards that he gained. I think that we're hopefully flexible enough offensively to try to take advantage of whoever we have in the game. Tim is certainly a good runner, so when he's in there, we'll probably let him carry the ball a few times."
Belichick also touched on Tebow's effectiveness as a running quarterback when operating out of an option-type attack.
"Tim has had a lot of experience making those decisions -- whether to give the ball to the back or keep it, or pitch it, all those kinds of things. It's not really like we're trying to teach him those things. He's done it a lot. He has to refine the timing and so forth, but it creates just another thing to put pressure on the defense," Belichick said.