Over at the U.S.S. Mariner site, Marc W. wrote an excellent essay about bullpens. It's entitled "The M's Bullpen Bet" but it's only a little about the Seattle Mariners and mostly about bullpens in general. He writes:
This is a gross oversimplification, but I tend to think baseball and baseball analysts have two primary views of relief pitching. The first is leverage: sure, they don’t pitch as many innings as some random 4th starter, but the innings they DO pitch are often critical to wins and losses. A great reliever pitches effectively in high leverage situations, and thus helps his team win close games.