The playoffs might not be "magic," but they're certainly different. During the season, you will be able to get a large sample set, so individual game performance fluctuations will average out, and the better player will have better numbers at the end of the year most of the time. In a win-or-go-home short series, that's not necessarily the case. In any one game, any one player can outperform any other player.

Of course, you should always start with the premise of putting the most talent on the field, so in general sticking with what got you there (as Flood Pants noted) would be the best starting point. However, that's based on the premise that we've "optimized our roster" this season, which isn't necessarily the case (see Hairston vs McGehee). Also, there can be other variables in effect. For instance, a somewhat minor issue which wouldn't effect decisions during the season may elicit more concern in the playoffs. Maybe Weeks is still hurt enough that it's effecting his game. Maybe nerves are effecting some players. Maybe Marcum has "hit a wall" with innings pitched. These could be things the managerial staff has access to which we will never know.

Also, all other factors staying the same, it may make some sense in a win-or-die situation to go with the variable with the least fluctuation, which is probably defense. In that case, it was a terrible decision to put Kotsay in CF, when the offensive difference among Kotsay, Gomez and Morgan was a toss-up, but the defensive side was a no-brianer that Kotsay was the worst option. In valuing starting Gomez/Morgan vs. Morgan/Hart or Gomez/Hart, you'd have to weigh the probability of each player helping the team offensivly and the probability of each player hurting the team defensively in one game... not an easy task. Over a season, it's no doubt that Hart should play nearly every day. Without any knowledge of the players' physical and mental states, it's very hard for an outside observer to make an informed decision.

Personally, it wouldn't bother me if Roenicke posted the roster PEM listed in post #1. At least I could understand the reasoning, which I can't say about last night's "go by the gut" roster.