I like Mallet and I hope he stays on the Pats. I don't want them to trade a QB that is competent and knows the system/team this close to Brady's senescence. I don't think a trade makes sense, unless it's for a high pick (which IMO is doubtful).
Brady's signed through 2017 with a cap hit that remains relatively unchanged each year (between $14-15MM). With the way the NFL is today, doesn't seem likely that QBs are in a ton of danger, especially the smart ones like Brady and Manning who know how to avoid hits.
If you're Ryan Mallet, you can re-sign for back-up money, hoping Brady actually hangs them up in 2017-18, and then potentially you get your shot at that point, perhaps with a team in cap hell from making runs in the next few years while the window is open with Brady. OR, when you're a FA after this season, you go sign elsewhere with a shot at starter's money and a chance to make a name for yourself. Which would you choose?
Mallet has very little incentive to sit behind Brady until 2018, at which point he'll be pushing 30 (he'll start this year at 26). And what's to say the Pats actually give him a shot then instead of drafting a younger, cost-controlled player?
Either the Pats cash in this offseason for a 2nd or 3rd+, or they believe the value of having a back-up to Brady familiar with the system is sufficient utility for letting Mallet walk and hoping they net a mid-late round pick from the NFL.