So the above are the Mets’ individual numbers against left-handing pitching this season. The chart is sorted by wOBA, Fangraphs’ everything-and-more offensive stat, for which .320-ish is league average and then higher is better and lower is worse. So among Mets regulars, David Wright, Scott Hairston, Andres Torres, Ronny Cedeno, and Ruben Tejada have all hit better than average against left-handed pitching this season, while Daniel Murphy, Josh Thole, Lucas Duda, Jason Bay, Jordany Valdespin, Kirk Nieuwenhuis, Justin Turner, and Ike Davis have all hit (pretty-far) below average against lefties.
The two players worth point out here are Jason Bay and Justin Turner, two right-handers who, unlike the Mets other right-handed bats, are not hitting left-handed pitching well this season. Breaking down those two more:
1. Justin Turner has a reverse-platoon split for his career, meaning that even as a righty batter, he hits right-handed pitching better. He’s got a .714 OPS against righties and .598 OPS against lefties for his career. I’m 85% certain Terry Collins has no idea about this, because he keeps pinch-hitting Turner against left-handed relievers and starting him at first base against left-handed starters.* (I forget about Turner’s splits all the time too, but I’m also not the manager of the New York Mets.) Turner’s rough numbers this season may not entirely be his fault. On the other hand, his roster spot may be better used on a right-handed bat who actually hits left-handed pitching, as otherwise Turner is just one more righty-masher on the lefty-heavy Mets.
*Turner has made 21 starts for the Mets this season, 16 of them against left-handed starting pitchers. I like Terry Collins and Sandy Alderson, but someone in the Mets organization should probably say something to someone about this.
2. Jason Bay had been terrible against left-handers (also against right-handers), but he does have a .129 batting average on balls in play in that split, which is extremely low. That number could be a product of Bay being terrible at hitting baseballs, or bad luck, or both. But Bay’s strikeout and walk rates against lefties aren’t awful, and he has some power against lefties. It’s just that his hits aren’t falling when he does put the ball in play. Bay is probably just done — he really doesn’t look like a Major League hitter anymore, nevermind a good one — but his poor showing against lefties may be more bad luck than totally being awful. If we’re looking for reasons why the Mets continue to run him out there while Duda and Nieuwenhuis hang out in the minor leagues.
3. Okay, there’s a third point: Why didn’t Vinny Rottino end up as the Mets’ right-handed hitting catcher/utility person? He must be really bad defensively (or insulted Sandy Alderson’s dog or something), or the Mets must really love Nickeas’ and Johnson’s game calling. But it seems like Rottino could help as a third catcher/right-handed bench bat. There must be something I don’t see about him, because Rottino plays a bunch of positions and seem like he could be league-average hitting against lefties. And I’m pretty sure that has value to a Major League team.
Patrick John Flood (blog) 
Vinny did a decent job for us this year and being able to play a little 1B and LF as well as emergency catcher is a pretty good use of a roster spotbut realistically he 32 and has 13 lifetime hits so far in the Majors.
Cedeno and Torres though are hitting lefties like first basemen and corner OFer’s. Imagine if a couple of Ike, Duda, Kirk, or Bay were too.
It was a bad move at the time they DFA Rottino and kept 34 backup short stops around. But the individual choice to keep Turner over Rottino was extremely questionable each time it was made this year (2 or 3?) Cedeno has taken over the RH 2B time lately, which was well overdue.
Along with a RH hitting everyday corner OF, they could definitely use a RH hitting backup IF. Lutz and Satin will probably get good looks next spring, Jefry Marte has to be in the conversation too, tho a long shot and only plays 3B/1B.
Campbell if he can come through would be a big help platooning at 1B and LF as well. Lagares might even get a chance at some point next year.