It's easy for a manager to over manage. Most of the time, it's best to let your starters play. They are your starters for a reason, so assuming you haven't made bad line up selections substitutions usually mean taking the bat out of a better player's hands. One area where a manager must make substitutions is with pitcher substitution. Not only must the manager know when his starter has had enough, he must pick the right pitcher to follow him. (And, without the DH, decide when it's advantageous to trade out even an effective pitcher for a pinch hitter).
The manager has control of a couple of factors. He can pick when he makes the substitution and who he puts in. This allows the manager to play the platoon match-up based on the upcoming batters. Having a lefty and a righty available in the bullpen means that, for at least two of the next three batters (barring switch hitters), he has the (very real) platoon advantage. And if you're starter is tiring, you can use every advantage you can get.
I don't mean to say that each pitcher should be a one out specialist, but rather given an all time great lefty and an all time great righty in the bullpen, picking the pitcher with the platoon advantage against more upcoming hitters makes good sense. And since I have my pick of the all time greats, I might as well take advantage.*
* And one more tiny thing: if my opponent knows I play the platoon advantage, he may feel compelled to always splitting his batters left-right-left-right. As dangerous as Williams-Mays-Ruth-Pujols may be, it's not quite as dangerous as Williams-Ruth-Musial-Mays, particularly against a right handed pitcher.
So, without any better justification than the above, I decided that my last bullpen slot should be a left handed pitcher. Well, that and there are some amazing LHPs to choose from.
Selection: Lefty Grove
Arguably the best LHP ever, Grove curiously gets left out of a lot of greatest of all time discussions. Perhaps because he pitched during a strong hitter friendly era, perhaps because he predated the Cy Young award or perhaps because his 300 wins (note: this is in no way an endorsement of traditional pitcher wins as a useful statistic, it's not), didn't stand out as much against the backdrop of the prior generation's greats like Walter Johnson and Christy Mathewson (or the next generation's Warren Spahn) and their astronomical win totals. Grove's career was also "only" 17 years long, which hurts him some in the various counting stats
But for those 17 years, Grove was amazing. His lifetime pitching line: 3.06/3.36/69/77 (ERA/FIP/ERA-/FIP-) puts him among the greatest starters ever and trailing only Randy Johnson and the brilliant but brief Sandy Koufax for post 1900 lefties in normalized FIP and beating them both in normalized ERA.* His peak wasn't bad, either, with a 2.06/3.01/47/71 in his 1931 MVP season (that's an ERA less than half league average). His FIP suggests he may have been a bit lucky that year and prefers Grove's also excellent 1930 season (2.54/3.09/53/64) as a more standout -- if slightly less effective -- performance.
*The relative merits of FIP and ERA over an entire career are definitely up for debate. Both try to correct a pitcher's performance for the vagaries of defense to provide a truer measure of a pitcher's performance than simple runs scored against average. FIP is a better predictor of future ERA than ERA for most pitchers, but some pitcher's consistently out perform their FIP.**
**And then there's xFIP. There's always room for improvement, we're pretty much always wrong, but we can be less wrong
Here's a fun debate: What are better, the quotes by Warren Spahn or the quotes about him?
He was something like zero for twenty-one the first time I saw him. His first major league hit was a home run off me and I'll never forgive myself. We might have gotten rid of Willie (Mays) forever if I'd only struck him out.
I'm probably the only guy who played for Casey before and after he was a genius.
Regarding his encounters with pre and post Yankees Casey Stengel.
I don't think (Warren) Spahn will ever get into the Hall of Fame. He'll never stop pitching.
He's not in the film, but he's still our all-time favorite left-hand pitcher.
--The producers of Naked Gun
And of course, "Spahn and Sain and Pray for Rain"
The quotable, talented, immortal Spahn is considered by many to be the greatest lefty of all time. But... the stats don't bear that out. While a deserving Hall of Famer in my mind, Spahn is the personification of the Very Good Accumulator*, with a career pitching line of 3.09/3.44/84/94. Even his two exceptional years by ERA-, 1947 and 1953, look like years that combined very good pitching with vergy good luck (with FIP- of 82 and 75, respectively). Twenty one years as a starting pitcher is an amazing accomplishment in itself, but Spahn's consistent career is somewhat lacking in peak performance to make this team.
*Next time you think or hear "It's not the Hall of Very Good" or "He's just an accumulator" regarding a Hall of Fame candidate, ask "Well, what about Warren Spahn?"
At the moment, Kershaw trails only Randy Johnson, Grove and Koufax in the normalized ERA and FIP among modern left handed starters. While it's early in his career, Kershaw was deserving Cy Young winner last year with a 2.28/2.47/62/66 line. Time will tell if he ends up being an all time great, but if you're looking for the next great left hander, Kershaw is your best bet.