The thinking, I guess, is that Murphy's going to wind up at first base and Martinez in left field eventually, so why not bow to the future now?
Well, because that's why they call it the future. It's not clear that Murphy, right now, will hit enough to play first base every day. Sure, you could platoon him at first base and he wouldn't kill you ... but then who's going to play left field? Right: young Fernando Martinez. But it's not clear that Martinez, right now, will hit enough to play regularly in the majors, and at 20 he needs to be playing regularly (or close to that). Fernandez currently sports a .315 on-base percentage in Triple-A, and there's little reason to think he'd do better than that in the National League. Not right now. In two or three years he figures to be one of the game's bright young stars. Right now, though? Probably not.
Here's the thing: Right now, the Mets have a first baseman. Fernando Tatis has backed up Delgado all spring, and he's been fantastic. You thought last season's .297/.369/.484 line was a bit of a mirage? I did. Well, this season it's .358/.417/.566. No, I know that's only 60 plate appearances and he's not really that good, but Tatis' career line is .266/.348/.447, which is better than Murphy or anyone else at hand is likely to do, right now.
Right now, the Mets have one big problem (or two, depending on how you look at it): their corner outfielders are not hitting. Murphy's not doing much in left field, and Ryan Church is doing even less in right field. Both of them bat left-handed. So does one of their backups, Jeremy Reed (who is hitting, but can't be expected to keep hitting). Gary Sheffield bats right-handed, but isn't hitting (and can't be expected to hit, ever).
The Mets need a good right-handed hitter, and they don't seem to have one on hand (unless you think this guy is the answer). I can't help thinking that a good general manager could find someone, just floating around out there, who can mash lefties and play left field or first base adequately. The Mets can survive, right now, without a guy like that. But if right now turns into July and August, people might start asking questions ...