The matchups are beneficial if you have a lefty who is actually really tough on LHB. Mike Myers is pretty good, as was Scott Sauerbeck before he got injured, Eddie Guardado is another. But more often than not managers just throw the lefty out there because they want a lefty/lefty match up. Not all left handed pitchers are abnormally tough on lefties, just from watching the Red Sox I can tell you Alan Embree is equally as tough against lefties and righties. Felix Heredia is just plain bad, he's a terrible example of a LOOGY, the only reason he was in the majors this year was because he was under contract and knows how to throw a baseball in the general direction at home plate, which was enough to earn you a spot in the Yankee bullpen this year.
