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.
I get all that. I know that's what baseball has degraded in to. That's one of the reasons why we have so many soft ass pitchers in the big leagues. I just don't like it.