Durability from the Rays' rotation

Clearly, the Tampa Bay Rays are having an excellent season so far. They're now just a game out of first place (and the majors' best record) behind the Yankees. They are unique in that they've gotten starts from only five guys --- James Shields, Matt Garza, David Price, Jeff Niemann, and Wade Davis. They are the only team in the majors with this distinction. Next in line are the Braves, Cubs, and Marlins, who have each used 6 starters this year. On the other end of the spectrum, the Nationals lead the major leagues in starters used with 12.