MLB use of 6 man crews for the playoffs

[Dakota]

MLB is, AFAIK, the only professional sport that changes the size, and therefore the responsibilities, of its officiating crews for the playoffs. This has always seemed odd to me. If 6-man is needed, why isn't it used during the season? If 4-man is sufficient, why isn't it also used in the playoffs?

Those of you who call or have called higher-level baseball (say, NCAA through MiLB), what is your opinion on this. Here are some questions that have bugged me for awhile, and since I don't umpire baseball at any level, I can't answer them myself. So, I'm asking you guys here.

Are the 2 extra umpires worth the potential for problems caused by instant expansion of the crew?

Will whatever benefits there are of having U5 and U6 go away (or become even a net negative) once instant replay use is expanded? In other words, if they are going to review the calls that are in doubt anyway, why have the larger crew?

Why doesn't MLB put the playoff crews together for the last week or so of the season, and call up MiLB umpires to cover for them in their regular crews? This would give the crews a few games to work together in the 6 man system before the playoffs begin.