OK, sure, in that scenario, yes you could make an argument for keeping Marcum.
I can't make heads or tails of the new CBA rules with respect to free agents. If he's having a great season, and the Mets offer him a qualifying offer, would they get a draft pick if he signs elsewhere in 2014? Does that depend on how well he does?
On the other hand, if the Mets are out of it, it would seem trading Marcum for something is a relative no-brainer.