And if M. Fiensand says it, it must be true. Matt Diaz is a long-time National League platoon outfielder, primarily with the Braves, for whom he hit .299/.347/.449. He's a career .324/.364/.498 hitter against southpaws, but only .258/.314/.362 against righties. Those rates are Esperanto for "don't bother." As an outfielder, he's a DH with a glove, kind of like Raul Ibanez was.
Now, having said all of that, I want to tell you that almost none of the foregoing may apply. Diaz had a very good 2009, and that's the last time that "very good" has appeared next to his name. From 2010 through 2012 he has hit .250/.298/.369 with just nine home runs in 630 plate appearances. His splits over those three years were .281/.328/.426 against left-handers, and .210/.258/.294 against righties. It could be, as the Who sang, that the good's gone.
Diaz's 2012 ended in unusual fashion -- he had season-ending surgery to remove what was thought to have been a bat splinter from his thumb but turned out, after having been excavated, to be a palm frond that had stabbed him in the hand back in 2006.
Diaz wasn't able to grip a bat properly due to the pain from having the palm fragments in his hand, so it's possible that he bounces back to his old platoony self now that he has presumably (palmsumably?) healed up. Keep in mind, his 2010-2011 numbers against lefties were just okay--.285/.327/.431. As a point of comparison, remember that this year, the average right-handed batter hit .263/.328/.426 against left-handers.
The Yankees aren't taking a great risk on this one, but it does seem redundant given (as I pointed out earlier) the presence of Ronnier Mustelier, who might be able to do the same job for nothing. A battle in camp won't really prove much -- no 30 at-bats does -- but the money isn't guaranteed and it doesn't hurt to have some depth in case something goes wrong.
Besides, might as well keep our powder dry -- nothing says the team doesn't go out and sign Scott Hairston next.