Girardi's kept everyone in the bullpen fresh and effective, without overusing arms out there. I don't think you can point to one guy in that bullpen that has pitched more than he should have, and that was a problem under Torre. He'd fall in love with 'his guys' and they'd lose their effectiveness, while other guys in the bullpen would rust.
Edwar Ramirez is an example. He pitched July 3rd and July 6th in 2007, and then not again until July 20th. He got lit up in his 3rd appearance, and then didn't pitch again for another month. Stuff like that hasn't happened under Girardi, and that has been a factor in getting the most out of players.