If you believe that a Tom Brady injury would mean the end of New England's Super Bowl hopes over the next year or two that would put you in the majority. If it seems likely Brady is going to be effective for at least another three or four years, then what good is a valuable backup signal caller? If Ryan Mallett is worth a decent draft pick, he should be traded.
In 2008 Brady was injured during the first quarter of the first game of the Patriots season. Instead of making a trade or signing a veteran quarterback New England went with unproven and for the most part unknown Matt Cassel. Despite winning 11 games with Cassel at the helm, New England missed the playoffs. With Brady set to return Cassel was sent to Kansas City along with Mike Vrabel for a first round draft pick. That picked turned into Patrick Chung.
There are two great reasons to trade Mallett. First, he has value despite having yet to prove anything. There are plenty of teams seeking a quarterback, it is a weak draft for signal callers, and a team would rather take a high upside chance on Mallett than sign Matt Moore and know he is dependably average.
The other reason to swap Mallett is that he is of no use to New England. His contract runs out following the 2014 season. If Brady plays this year and next we never see Mallett, he goes elsewhere, and you get nothing for him. If Brady goes down, and Mallett plays well, at best he looks like Cassel, you still don't win the Super Bowl, and you either trade him or have a quarterback controversy that Brady will win. If Brady gets hurt and Mallett plays poorly, he loses all of his value.
The Patriots are in need of extra draft picks. They are without selections this year in the fourth, fifth, and sixth rounds. While Aqib Talib, who cost them the fourth rounder was an impact performer, Albert Haynesworth, and Chad Johnson were not worth fifth and sixth round picks.
In recent years backup quarterbacks like Kevin Kolb and Matt Flynn have had value and then failed to produce great results. If Mallett can net the Patriots a pick or two, he is worth dealing. What is the down side, he has a great year with another team and you still have Tom Brady? Unless New England is going to invest in Mallett to replace Brady, like Aaron Rodgers shoved Brett Favre out the door, his greatest value will come on the trade market.