The Yankees already have a full outfield. Brett Gardner, Ichiro Suzuki, Alfonso Soriano, and Vernon Wells. There's also Curtis Granderson to think about and they're interested in Shin-Soo Choo as well. That being said, David Murphy could still be an upgrade. If the Yankees release Vernon Wells, which they should if they're only paying him $2.4 million or so, Murphy could prove to be a better fourth outfielder.
Murphy had his worst season in 2013, directly after his career year in 2012. However, that could work to the Yankees' benefit because he could come cheap. This season he only managed a 73 wRC+ and he's put up alternating above and below average offensive seasons since 2008 and 2014 looks like his next up year. Not that it means much.
He would be a good left-handed bat to hit against righties because he has a 113 wRC+ against them. While replacing Wells with a lefty doesn't do much for the Yankee bench against left-handed pitching, Murphy could share time with Ichiro Suzuki, who couldn't hit righties, but was league-average against lefties. He's also a good fielder, with 17 defensive runs saved over the last three seasons. Bringing Murphy to play left field against righties would allow Soriano to move into the DH spot. Murphy also has some right field experience, so Ichiro doesn't have to play every day once again.
Coming off his worst season at the age of 32, Murphy could be looking for a small one-year deal in order to prove that he's worthy of a bigger deal. He made just over $5 million, so another contract like that could end up bringing a lot of value if he regains his form.
Unfortunately, bringing in David Murphy would complicate an already confusing outfield situation. The Yankees need someone who can hit lefties if they're going to replace Wells and if Granderson returns there already won't be enough room for him. Murphy would only really be a good signing if they decide to make Soriano the DH or they clean house and dump both Wells and Ichiro, but that is unlikely to happen.