At this point, I think he's worth a third-round pick or so. By drafting someone like Matt Barkley in the second round, you're getting a younger quarterback for four years, whereas with Mallett, you have him for two years. I also don't think indications out of New England are that he's seen as the future of the team.
Bill Belichick has shown a willingness to replace veteran quarterbacks with younger players before (Bernie Kosar, Drew Bledsoe), but opted to give Brady another three-year extension rather than let him play out the deal and reevaluate then. If the Patriots evaluated Mallett as a franchise quarterback in waiting (or even a quality starter), I doubt they would have committed to Brady.
If he re-signs with the Patriots, Mallett will be 30 years old by the time Brady's deal expires, at which point he could be a Brandon Weeden type, old for a first-time starter and without pro experience. There have been rumors that the Patriots are unwilling to trade Mallett, but I think that's posturing for a potential trade partner. Obviously you don't want to say "we have to get rid of this guy", otherwise you're not going to get value. It wouldn't be bad if Mallett stayed with the team, but I think this is a prime opportunity to trade him and replenish some of New England's missing draft picks. There are also a number of backup quarterbacks who have some familiarity with New England's coaching staff: Tebow with McDaniels, Cassel with McDaniels and Daboll, Quinn with Daboll, etc.