Ok, I know this horse has been beaten before, and sure it's dead, but that doesn't mean it couldn't use a few more good whacks.
It is possible that Joakim Soria's mechanics give rise to a significant injury risk if he became a starter. If true, then this is a valid reason for keeping him in the bullpen. But the modern traditionalist opinion is that shutdown closers are extremely important because they spread their influence over so many games. Noted good pitcher, but unspectacular analyst Gil Meche recently churned out another example of this philosophy:
I was actually thinking aobut that this morning. I was brushing my teeth and I was looking at the schedule -- I'd brought the schedule home and Soria was on the front of it. And I thought, 'I really wonder how he would be as a starter? The way he makes his innings so easy.' And then about 10 seconds later, I'm thinking, 'Well, hold on. To have a guy that can just dominate the ninth inning -- in a way, it's more valuable to have that. It's more important to have a guy nailing down games than maybe a starter winning 13 to 14 games.'
I have long opined that Joakim Soria would be more valuable to the Royals as a starting pitcher rather than as a closer, pointing out that a #2-quality starter contributes more to his team than even the most dominant starter. A number of stats and studies pointed in that direction, but at the time I didn't have the excellent total value stat Wins Above Replacement (WAR) at my disposal. My inspiration for this particular punch at ol' Seabiscuit is an excellent article at Lookout Landing about Brandon Morrow's recent assignment to the Mariner's bullpen. This article points out the win value of a dominant closer as compared to starting pitchers. I'm going to borrow from it, expand on some of the numbers and I think it will be clear how much more Soria would help the Royals as a starting pitcher.
Without going into a long, drawn out explanation, WAR is an excellent total value metric for pitchers as well as hitters. It takes into account meaningful stats like FIP or tRA, the innings that he pitches and the leverage of those innings. That last phrase means that WAR accounts for the fact that closer innings are more important innings.
So how many Wins Above Replacement does a dominant closer give to his team? I took arguably the five best closers in recent years, looked at their healthy, full seasons as a closer (since 2002, as this is all I have WAR data for) and averaged their WAR over those seasons. Here are the results:
Francisco Rodriguez (2004-08) 2.5 WAR
Brad Lidge (2004, 2005, 2008) 2.8 WAR Jonathan Papelbon (2006-08) 2.8 WAR Mariano Rivera (2003-08) 2.7 WAR
Brad Lidge (2004, 2005, 2008) 2.8 WAR
Jonathan Papelbon (2006-08) 2.8 WAR
Mariano Rivera (2003-08) 2.7 WAR
Joe Nathan (2004-08) 2.6 WAR
From this, it appears that the contribution of a dominant closer is about 2.7 WAR. [For comparison's sake, Soria was at 2.4 WAR in 2007 and 1.5 WAR in 2008, for an average of 2.0] How does that compare to starting pitchers? Here are some starting pitcher numbers from 2008:
In the AL in 2008, the average #3 SP had a FIP of about 4.40. I think the above numbers show that if a pitcher can start for a full season, eat a decent number of innings and pitch at a roughly average, #3 SP level, he will account for around 2.7 WAR. Starters who pitched at the #2 level for a full season (like Weaver, Millwood and Sonnanstine) had WAR's in the 3.4-3.6 WAR range. Therefore, if Soria could pitch at the level of a #2 SP (even a below average #2), then he'd be helping the Royals more than if he continued to develop into one of baseball's most dominant closers. And then there's the upside potential that Soria could become an above average #2 (like Gil Meche, averaging 4.5 WAR over the last two years) or even a legitimate #1 (the sky is the limit, WAR-wise).
So would you rather have Soria in a role where he would top out at 2.7 WAR if he's one of the very best in baseball in that role, or in a role where he could easily manage 3.5-4.5 WAR and has the potential for more than that?