This will never cease to confuse me. What good is a pitcher that can only be effective against batters from one side of the plate? Guys like that shouldn't be in the majors in my opinion.
If you can find a guy who can get both types of hitters out with equal efficiency as a lefty specialist gets out lefties, you would have gold. The reality is that teams keep lefties around in the bullpen for situational purposes. It may not seem useful on a sub-500 team, but it is difficult to make a playoff run or win a title without one or two of them.