The Importance of Depth

In Baseball Prospectus' new book, It Ain't Over 'Til It's Over, the first chapter talks about the importance of having one great player on a mediocre team to having a few good players. They raise two questions:
"1. How much impact does the presence of one great player have on a team's chances[to make the playoffs/win WS]?
2. How much impact does the presence of one great player have on a team's chances if he's head-and-shoulders above all his other teammates?"

The answer to the first question is that most great teams had at least one GREAT player, meaning that a playoff team with no outstanding player is basically a fluke. The answer to question number two is that a great player helps his team immensely but the more talent the team has the better it does overall. They show this through correlating a team's best player's WARP(Wins above Replacement Player, accounts for all facets of the game) to winning percentage and a team's second best, third best, etc. to winning percentage. This shows that a great third or fourth or even fifth best player is more important than a team's best player over the course of a season because baseball is a team sport so the better the supporting cast the better the team is. The correlation between a team's number one player's WARP and winning percentage is .49(it works that 0 means no correlation and 1 means total correlation), but the correlation between the team's fifth best player's WARP and winning percentage is .69 a significantly higher number.

This shows that the lower down you go on a team's depth chart the more important the player is to your team's success(winning percentage). The Red Sox '07 championship team is paradigmatic of this principal. They had Coco Crisp, an average CF, on the bench through the playoffs and now, during this season. If you look at the Red Sox they have no holes whatsoever. Kevin Youkilis is better than most teams second best player and he's their seventh best player(before him are Ramirez, Ortiz, Beckett, Dice-K, Papelbon and JD Drew) and that's just ridiculous. A great player or two might be able to carry you for a month or so but having a great core of players can lead you through the season successfully and maybe even all the way to the promise land like the '07(and '08) Red Sox.