As fans, we often find things to criticize in our team's on-field management. As analysts, we do the same. Sometimes these criticisms are warranted ("Now batting leadoff, Corey Patterson"), but sometimes they are misguided rants born of frustration. Moves don't always work out, but that doesn't mean that, given the information available at the time, it wasn't the right decision. One common situation that tends to raise a lot of unnecessary ire is the usage of a bullpen in platoon situations, particularly regarding how left-handed specialists are used.
The situation generally plays out in some form of the following variation: a lefty comes in to face a left-handed hitter, gets him out, and then is left in to face a right-handed hitter, whereupon the clamouring begins and continues to all hours of the night, and often into the next day, or through the next day if it is a travel day, in living rooms, press boxes, stadium seats, bars, messageboards, etc., crying for the manager's head, or about the manager's head, by people who believe they are in the manager's head. In these discussions, you will often hear things like: "He's fine when he's used properly, but you can't let him face righties." The assumption here is that proper usage of a left-handed specialist would be to face very few, if any, right-handed hitters. The problem with this assumption is that it is made without a sufficient understanding of context.
Roster spots in MLB are limited commodities. As such, substitutions have to be used wisely. Managers simply don't have the option to switch pitchers every time a change would create a more favourable match-up. What is best in the context of an at bat is not always what is best in the context of a game, let alone what is best in the context of a 162 game season. As such, the ideal of a left-handed specialist who only faces lefties and makes appearances 1 or 2 hitters at a time until a righty comes up is little more than a myth. That perception, while deep-seated in the baseball community, is unfounded. Those lefty-only pitchers don't exist as people seem to think.
Since 2000, there have been 73 times (about 8 pitchers a year on average) a pitcher has faced more left-handed hitters than right-handed hitters in at least 25 IP. There have been 30 where a pitcher has faced more than 55% lefties. Only 4 times has a pitcher faced fewer than 40% right-handers (2 of those were Mike Myers, the closest thing to the myth we've seen, who is also the only one to top 62% lefties faced in at least 25 IP).
There have only been 19 pitchers since 2000 who have faced more lefties than righties in at least 25 IP in more than one season, and only 5 who have faced more than 55% lefties multiple times. Only 13 pitchers with at least 50 IP have faced more lefties than righties in their careers since 2000, and only Jesse Orosco (from age 43 on) and Mike Myers have faced more than 54% lefties. Forget pitchers who face almost exclusively left-handers - pitchers who regularly face a majority of lefties year in and year out are rare, and almost everyone who is a regular roster member for any length of time ends up at least close to even on the number of lefties and righties he faces. Short-term or September call-ups are one thing, but if you are going to give a pitcher a permanent roster spot, he will have to bear at least a moderate workload (at least 40-50% of all batters faced in most cases, and usually more) against opposite-handed hitters. To expect that not to happen is not reasonable, given historical usage of left-handed specialists.
Additionally, pitchers can't get away with only facing one batter at a time throughout a season. Since 2000, Jesse Orosco is the only pitcher to make at least 25 appearances in a season and average fewer than 2 batters faced per appearance. Eighty-two (about 9 per year) times has a pitcher faced fewer than 3 batters per appearance in a season with at least 25 games. Twelve pitchers with at least 50 career appearances since 2000 have faced fewer than 3 batters per appearance, and only a third of those have 150 career appearances since 2000. Unless opposing teams are regularly going to run out multiple lefties in a row without pinch-hitting for any of them, your left-handed specialist will have to stay in to face right-handers a lot of the time.
There are, of course, times that managers should play the matchups. In high leverage situations, the value of a single plate appearance can justify the burning of a pitcher. When a bullpen is particularly rested, or when fresh call-ups are stocking the pen, or late in games when 1 pitcher has born most of the load, the cost of a pitching change is lower, and managers can afford to play more matchups. In September when rosters are expanded and teams in pennant races need every advantage they can get, or in the postseason where there is extra rest between games and each at bat is critical, there is little reason to conserve pitching changes at the expense of your chances at getting the out. However, as fans, we need to realize that in most situations over a season, there are factors to be considered beyond the present at bat, and often times those factors are simply more important than getting the best possible matchup in every plate appearance. In the case of permanently rostered left-handed relievers, that means that they have to face righties. There isn't any way around it.
What a left-handed specialist means in today's game is not that a pitcher does not or cannot face right-handed hitters. It means that he is among the handful of relievers who face more lefties than righties, or is at least close to 50-50 between them. Much beyond that, while often the expectation of fans, is not reasonable. I have no idea why such a strong perception otherwise exists when it has never been true. I first noticed how many righties left-handed specialists faced when writing another article on left-handed specialists late last year; at that point, I was shocked to see the pitchers I was looking at facing so many righties. This offseason, I read an article on a well-known statistical website by a respected writer calling out Ron Gardenhire's usage of Denys Reyes for facing so many righties when he was in fact among the lowest in baseball in percentage of right-handers faced. With such an ingrained idea, I can't really blame fans for not realizing that it is so unfounded, but it is this type of lack of awareness of proper context that leads to far more distress among fans than is called for. One of my goals with this blog is to try to establish a stronger sense of context for areas of the game where such is lacking, and this is one issue that jumped out as being in dire need of context. So hopefully next time you see your manager leaving a pitcher to face in a situation he seemingly doesn't belong in, you can have a better idea of why.