Left hand pitchers have always been assumed to have an advantage because there are much less that throw from that side. Conventional wisdom shows that many pitchers have had their careers extended due to the demand for left hand relief. In addition to starters who have extended their careers, former closers like Jesse Orosco, Mike Stanton and Dan Plesac had as much if not more success as lefty specialists. Being a LOOGY allowed for pitchers Sparky Lyle, Willie Hernandez and John Franco to pitch a couple more seasons.
However, the change did not work for all starters. LHPs Mickey Lolich lasted just two more seasons and was done pitching by age 38. Jimmy Key retired in 1998 after pitching as a left handed specialist for the 2nd half of the season for Baltimore. And Mike Flanagan had 1 solid season as a LOOGY followed by a terrible one for the Orioles. Ask Mitch WIlliams how he adjusted to the lefty specialist role. Al Leiter struggled pitching in that role in the 2005 postseason for the Yankees.
So what it means is that not every left handed pitcher can move into the role. A current example is Mets LHP John Lannan, who is probably better suited to be a long reliever or a spot starter. Chris Capuano is an extra starter for the Boston Red Sox. I am sure it will be assumed he can get thrive in the role as lefty specialist, simply because he is a left handed pitcher. The Red Sox would be wise to not explore that possibility. Billy Wagner is an example of a reliever who never took on that role and hopefully Kansas City LHP Greg Holland and Cincinnati LHP Aroldis Chapman never take on that responsibility. Maybe they might to prolong their careers.
What is the worst part of this assumption is that teams are doing this to kids at lower levels of the minor leagues. First of all, kids that end up only facing left hand batters in A ball have about 0% chance to make the big leagues. Second, hand throwers who are having success as a closer get brought up to the majors in a role they have never had before. For example, the Mets rewarded LHP Josh Edgin with an opportunity to pitch in the big leagues in 2012 after pitching very well at different levels. Unfortunately, he had little experience in the specialist role, something he had to learn in the big leagues. As he starts 2014 in the minors, it is a wonder whether the Mets will use him in lefty batter only situations or allow him to pitch entire innings. The same can be said for LHP Rob Carson, who is now with the Los Angeles Angels organization.