When you have Mariano Rivera in the bullpen, it's perfectly fine to stick with one closer. Over the years, as teams have tried to duplicate the success of teams that had dominant closers such as Dennis Eckersley, Bruce Sutter and Rivera, they have become more and more dogmatic about roles, and that can be to their detriment.
In the postseason, you need to do what you need to do to survive. We are seeing that this October, as in Detroit, Phil Coke has stepped in for a struggling Jose Valverde, and in San Francisco, Giants manager Bruce Bochy has remained flexible, closing out games with both Santiago Casilla and Sergio Romo.
History is littered with examples with teams who weren't slaves to the dogmatic notion of who was the closer with the capital "C." In fact, that is a notion that has really taken hold only recently.