Each major league team is allotted 25 roster spots at any given time. Any combination of hitters and pitchers is legal. Up until the 1990's it was common to see a team carry just 10 pitchers (often less), allowing flexibility on the offensive side, while relying more heavily on 'ace' relievers to come in for multiple innings at a time if the starter couldn't finish the game. Needless to say, that has changed a good amount in the past 20 years, much to the chagrin of many baseball analysts (and myself <--- not an analyst). The waste of carrying about 12 pitchers is fairly evident if you think about it. Those extra 2 or 3 pitchers are almost certainly not of the highest quality, and are taking up innings from more effective pitchers. For example, on the Twins, pitchers such as Matt Maloney and Jeff Gray are getting significant time on the mound. The innings would be better off with Glen Perkins, Brian Duensing or even Matt Capps, and the roster spots would be better of given to a competent third baseman (Trevor Plouffe got the start tonight) like Sean Burroughs or Luke Hughes who were essentially waived in order to maintain the bloated bullpen. This strategy often takes roster spots away from good hitters like Hideki Matsui or Johnny Damon, who only recently found jobs, or versatile defensive specialists are useful in many late-game or pitcher-determined situations. Now, usually an example like the one I used in the Twins, would be would be used as an argument against the 13 man pitching staff. However, in this case, I'm not so sure.
I'll try to make this short, but the Twins starting pitchers suck. A lot. The best starter ERA is 4.91, belonging to Carl Pavano. Only Pavano has completed the sixth inning more than twice this season. The average length of a Twins' start this year? A tick above 5 innings. Given this suckitude (suckosity?), relievers have been counted on for about 4 innings a game, or at least 96 innings over the 24 game schedule to date (that's without adding in extra inning games). Extrapolated out to a year, that's about 650 innings of relief. To put that in perspective, even if the Twins carried 8 relievers, each would have to pitch 81 innings. Then to put that in perspective, only six relievers in the entire MLB through more than 81 innings last year. As you can see, given the massive workload put on their relievers, carrying 13 pitchers is a necessity as a bare minimum for the Twins right now. I don't like to see that, I don't like to say that, but with the state of the Twins currently, it's just true.
Clearly something needs to change, whether it's regression of the starters back to more acceptable production levels or Gardenhire forcing the starters to take on more of a workload. The modern reliever is not built to be effective at 80+ innings a season (that's up for debate, but is a whole different issue), and the Twins will see that soon. In the meantime, we may be witnessing one of the first necessary 13 man pitching staffs in history.