You might have wondered, after the botched/not botched (depending on your view) call by left-field umpire Sam Holbrook in the Cardinals/Braves wild-card game Friday, why there are six umpires in postseason games, when we do perfectly well (most of the time, anyway) with four of them during regular-season games. SABR's Larry Gerlach explains how we got to that six-man crew:
In 1947 the "alternate" umpire from each league, who had been seated in the stands from 1940 to 1946, was stationed along the foul line in the outfield thus establishing the now-familiar six-member Series staff.
The six-man staff, of course, was later expanded to all postseason games; from 1947-63, though, the "alternate" umpires remained in the outfield, not taking part of the usual umpire rotation. Sometimes they kept their left-field or right-field slot for the entire Series; sometimes they swapped positions. In examining World Series boxscores from those years, there didn't seem to be any systematic way in which this was done, probably because those positions were seen as an afterthought. In 1964, though, these extra umps were made part of the rotation, and that has continued to this day.
The first relevant question is "Why was this done in the first place and why is it done now?" The answer to the first part of that question is lost to the mists of time; it seems likely someone thought, "We're paying these guys to come to the World Series, let's give them something to do." And after 17 years, they decided to institutionalize it and make the "alternates" an official part of the crew.
The second, and even more relevant question, directly connected to what happened in Atlanta Friday, is "Why is Major League Baseball asking umpires to do something in the postseason that they never do at any other time?"
That's not entirely true, of course; we have six-man umpiring crews in the All-Star Game and that's been the case since 1949. But that's just one game, and despite "This Time It Counts", it's not nearly as important as postseason games.
This isn't the first time that a call by an outfield umpire has potentially affected the result of a postseason game. In 2009, left-field umpire Phil Cuzzi called a drive down the line by the Twins' Joe Mauer foul, when it was clearly fair. Even SB Nation's Yankees blog couldn't believe the call:
It's inexcusable for a professional umpire to miss a call like that. I've seen missed calls on baseballs that hit the line or that were inches fair, but not five or six feet fair. I don't know what Cuzzi was watching, but it couldn't have been the ball. I watched the replay again about a dozen times and there's not even a logical explanation for what he could've seen.
"Foul-line umpire" isn't a position that is manned by an umpire -- except in the postseason. Umpires must make calls quickly and accurately; a significant part of doing that is knowing where to position yourself depending on the game situation. There's really no logical reason to have an umpire down the foul line, when the base umpires can make these calls more accurately than someone who might literally never have been in that spot before.
Solution: Go back to four-man umpiring crews for all games. The foul-line umpires are an historical anomaly that's done more ill than good.