I realize this is a quite strange question for this time of year (blame video game baseball, although all the discussion around the Carp trade and its potential impacts vis-a-vis the 40-man is what directly reminded me) but why are the rules around who can play in the post-season so complicated?
I understand the idea of barring players acquired after a given date--otherwise, even with the non-waiver trade deadline in place, it would be too easy for teams certain of their playoff fates to radically shift at the last minute. What I can't conceive of a reason for, though, is the requirement that on that date an eligible player be on the 25-man roster rather than just the 40-man. By placing someone on the 40-man, a team has already made the decision to sacrifice flexibility and money in order to have them available for major-league service. Why shouldn't they then be able to use such a player--a player with a major-league contract--to fill the unique balance of needs created by a playoff series?
(For instance, consider a team with a good 5th starter, a so-so long reliever, and a hot-hitting prospect at AAA who's on the 40 man. Since a playoff series only requires 4 starters, it would make all the sense in the world for this team to move the 5th starter to the long relief role, and use the extra spot to add the prospect as a bench bat--but, unless I am grossly misunderstanding things, the rules prevent this.)
I'm certain there must be some kind of good reason for this, I just can't think of it. Does it have something to do with the rules around service time? I confess to not being entirely well versed in those. I'm only a relatively recent convert to what (for lack of a better word) I'd call analytic baseball fandom, mostly thanks to this blog, so I thought that one of you might be able to explain it to me.