Pitching depth means having Clemens to pitch three innings of relief in Game 4, since Pettitte was fresh for Game 5 -- as opposed to having to sit on Smoltz to save him for Game 5, and not having a solid starter who could eat innings effectively in the longest playoff game in history.
It also means, even coming out of a meatgrinder like this, having Pettitte, Oswalt, Clemens, Backe, Pettitte, Oswalt, Clemens lined up for the NLCS.