When it comes to baseball, I believe the tenets of the sabermetrical approach are holy...except for relief pitching. I have watched enough guys blow leads in the 8th and 9th innings to realize that the last 2 frames of a game are the toughest to have success. I know many sabermetricians believe that you can run a bullpen by committee and it can be successful. I don't. Some guys have the sack to do it and others don't. Losing a lead and then the game in the last 2 innings are worse than losing them in the first 7. I know mathematically this makes little sense, as a computer sees a loss as a loss. Well in my time watching the dynamics of a team, blowing games at the end wrecks a team's confidence faster than anything else. There have been plenty of great hitting teams who couldn't get over the .500 mark. While it is less the case, the same could be said for teams who had a better than an average starting pitching, I would argue that over the past decade, if you had a top-notch bullpen, you were going to be a playoff competitive team at worst and most likely a division winner or better.
The 2 top bullpens in the AL are the Red Sox and Angels, which I believe to also be the best 2 teams in baseball. Sure they have excellent starting pitching and hitting, as well, but it is their ability to close-out the last 2 or 3 innings that put them ahead of their competitors. The Indians went from being underachievers to nearly winning the AL Championship because their bullpen had an inspired year in 2007. I think the Indians are the best team in baseball, if you don't include the bullpen, but a team which counts on Joe Borowski to get them through the 9th inning is insane.. Another bullpen that pitched out of their minds in 2007 was the Mariners. I suspect they will come back down to earth this year, which will make them a team who struggles to be much more than a .500 club.
When you judge teams over the past decade on the category of who has had the most results on a cost/production scale, the A's and Twins battle each other for the top spot. While they have used very different approaches on building competitive clubs, the one thing they have shared is possessing excellent bullpens. Sure they've had good starting pitching and defense, but it was the consistency of their relief pitching that was their unsung hero. The Angels have also been lights out from the 7th inning on, givng them a leg up on every team the past few years. I would argue that the MVP of the Yankees over the past decade has been Mariano Rivera. Having the greatest closer in baseball history has been the stabilizing force for the Bronx Bombers, when the rest of franchise was going through one media-hyped crisis after another.
Many touted the Tigers as being right there with the Red Sox and Angels, but their bully was a huge question mark. Remember the days when Todd Jones was the 3rd or 4th best option for Detroit, even though he was their closer? Not anymore, as he is the best healthy relief pitcher they have.
Name me a American League team that was a playoff team without a good bullpen during this decade? The only one I could find was the 2003 Red Sox. Remember them, the team that followed the idea that closer by committee would work? Even with Bill James in the front office, the Red Sox quit that idea after one year, bringing in Keith Foulke the following season. I know that you can't make a concrete conclsion off of one example, especially by a team that had Byung-Hyun Kim as their top save leader, but I'm telling you closer by committee won't work. Some guys can't handle the final 3 outs, some can. You can say it is BS, but ask major league relievers if they don't perform better when they know what their roles are in the pen.
Put this last statement under the category of please do the research for me, but my guess would be that teams that strongly under perform or over perform their Pythagorean record are almost always most affected by their bullpen. If you look at my pre-season picks, you will notice that bullpen performance is the biggest factor why I think a team will have final success. This is why the optimistic predictions of a winning season by the Rays I find very unlikely. That's all I have for now.