Here is the thing. I've railed about the "relief ace" idea. But we are a decade away at least from that getting traction. The groupthink behind closers -as well as the way that salary arbitrators use the statistics for compensation - the closer is going to be the best compensated reliever. Managers, even the Joe Maddon sorts, fall into line with this stuff - that the 9th is special etc. There is some risk aversion at work - the same things that prevent football coaches from going for it on 4th down when anybody who has played Madden knows better. Nobody wants the media or fans getting on their ass, easier to deflect to players than to take the bullet.
SO, with that being said, closers now are essentially like how running backs have evolved in the NFL. You need a running back, but it doesn't matter as much who he is. He can be Stevan Ridley, Shane Vereen, whomever. The team structure is more important. Now there are a few RBs (like closers) who can do a heavier lift - THOSE guys, yeah, are real value adds. The Red Sox in their three title runs were blessed with true value-added closers, in Foulke, early Papelbon, and Uehara last year. Value-added closing is very hard to find, and Rivera was one of them. But Bobby Jenks wasn't, and Sergio Romo isn't, and 2006 Adam Wainwright wasn't.