Left-handed relievers lead a charmed big league existence. They work fewer innings than right-handers, they’re coveted every season at the trading deadline, and they earn multiyear deals worth millions of dollars to get a couple of outs a few times a week, even if their past results don’t justify such rewards (see Schoeneweis, Scott). Not only that, but they have more job security than any other group of major leaguers for, due to their scarcity, lefties can hang around a big-league bullpen almost as long as they see fit — Jesse Orosco, the ultimate situational lefty, pitched until he was 46.

Take the example of the newest Yankees hurler, Damaso Marte, who arrived last week in trade from the Pirates. Marte was one of the most desired assets on the market this July, despite the fact that he has not thrown 60 innings in a season since 2004, has not cracked 80 in any year of his career, and has amassed only a handful more saves (36) than blown saves (25) overall. But New York did not have an established left-hander in its bullpen, and felt pressure to add a pitcher who could effectively attack lefty sluggers toward the end of games.

Does a team really need a southpaw in the bullpen to be successful? In the abstract, the answer is no, as proven this decade by the Angels. In 2004, Anaheim won 92 games and the AL West while getting two innings from lefty relievers all season, the second-lowest total for any team in the last 50 years (the ’04 Brewers had zero left-handed relief innings). Despite that imbalance, the ’04 Angels’ bullpen finished second in the American League in expected wins added above replacement level (WXRL). The 1984 Cubs provide a similar case study, as they took the NL East with 96 wins despite getting all of 6 1/3 left-handed relief innings.

Not having a lefty in the pen, however, is different from not having anybody who can get left-handed hitters out. While the former is not a necessity, the latter definitely is, especially for a team hoping to succeed in October, when individual late-inning matchups are intensely magnified in terms of the stakes dependent on their outcomes. The 2004 Angels had three pitchers — Francisco Rodriguez, Scot Shields, and Brendan Donnelly — who held lefty batters below a 650 OPS (on base plus slugging percentage). Shields was a lefty assassin that season, posting a “reverse” platoon split by holding them to a 610 OPS, compared with 665 for right-handers.

The Yankees also have a reliever who famously exhibits a reverse platoon split, closer Mariano Rivera, whose equally famous cutter bores in on the hands of lefty batters, making their task against the future Hall of Famer nearly impossible. Baseball Prospectus’ PECOTA system projected just five right-handed pitchers currently active in the majors to have better performances against lefties than righties this season. (Rivera was joined by teammate Mike Mussina, the White Sox’s Scott Linebrink, the Cardinals’ Joel Pineiro, and Boston’s Tim Wakefield.) New York’s stopper has made good on that forecast, as lefty batters have hit an absurdly low .163 AVG/.182 OBA/.233 SLG in 89 plate appearances against him, compared with .177/.214/.266 in 84 appearances for righties. Over his career, Rivera’s right-handed opponent OPS is 602, but an even lower 522 by lefties.

Besides Rivera, a stealthier on-hand option for the Yankees could be 31-year-old rookie Dan Giese, who between Triple-A and the majors from 2007-08 has allowed a .188/.238/.300 line to lefties in 315 plate appearances, versus .249/.284/.370 to righties in 359. Another Bombers bullpen rookie, Edwar Ramirez, has also performed well versus left-handers; Ramirez did not let them bat higher than .170 at any of his three minor league stops since arriving in the organization, thanks to a devastating changeup.

But that is not to argue against the team’s acquisition of Marte. The fact is that manager Joe Girardi does not yet trust either Giese or Ramirez to pitch in critical spots — in terms of Leverage, a Baseball Prospectus stat measuring the importance of the situations in which a reliever is used, the pair are at the bottom of New York’s current bullpen depth chart, despite the strong work done by each. Rivera is used primarily in the ninth inning, only occasionally coming into the eighth. Before Marte’s arrival, that left the recently departed Kyle Farnsworth and Jose Veras as the two primary setup men, and both Farnsworth and Veras are worse versus left-handers, Farnsworth particularly so this season. Getting Marte, against whom opposing lefties have hit .200 with a 581 OPS lifetime, therefore adds an established arm Girardi can trust to get lefties out long before the game gets to Rivera.

Marte’s acquisition makes particular sense when considering New York’s late-season quest for a playoff berth. The Yankees still have six games remaining with Boston, including the last three of the year, which could very well be the deciding factor in whether or not New York extends its record streak of playoff appearances to 14 straight seasons. Nobody is ignorant of the fact that for lefty power, Boston’s lineup boasts both platoon monster DH David Ortiz (career OPS split of 980 vs. right-handers against an 831 mark vs. lefties) and right fielder J.D. Drew, who has a similarly big split (926 to 799).

The team against which Marte could prove to be even more useful, however, is the one still in first place in the AL East. New York also has six games left against Tampa Bay, in which there will likely be plenty of chances for Marte to prove his indispensability. That’s because the Rays have gotten a greater ratio of their plate appearances this season from left-handed hitters (58.2 percent) than every other American League team other than the Yankees (60.3). The Rays have two switch-hitters, but several of their big bats — Carlos Pena, Akinori Iwamura, Carl Crawford, Cliff Floyd, and Eric Hinske — hit strictly from the left side, and can potentially be neutralized by a southpaw of Marte’s caliber.

New York probably had more to gain than any team in the game by adding a left-handed reliever, given its previous lack of one and the tactical value a lefty holds against the Red Sox and Rays. It’s not that they couldn’t survive without one, as the 2004 Angels show, but between Marte, Rivera, and perhaps also the under-utilized Giese/Ramirez combo, the Yankees now should have enough pitchers to neutralize left-handed batters to win some pivotal late-game matchups in the season’s final months.

Mr. Peiffer is a writer for Baseball Prospectus. For more state-of-the-art commentary, visit baseballprospectus.com.