An reliable pen for innings 7-9 always makes a difference, but you get that with a good closer as much as some so-called elite closers. We saw that first-hand with Coco. He converted 86% of his save chances with the Reds, which is very good.
An elite closer might get you an extra save or two over the course of a year (though some of that is dependent on luck) and an extra win could make all the difference. But it's not a terribly big gap between the best closer baseball and an average closer. And most elite closers tend to have short stays at the top. Rivera, Hoffman and Wagner were freaks. More common are guys like Joakim Soria, K-Rod (already on the downhill when he saved 62 games) and Brian Wilson.