It all comes down to what goes into to determining the value of a player. If you're just a stat geek, then sure, you're gonna say "Mariano was the best reliever of all time, but nowhere near as valuable as a starter or a position player who played so many more innings." And there is some truth to that. But if you're taking everything into account - Mariano has been almost lights out his entire career, and with only 1-2 pitches. Plus he's performed in one of the most high-pressure cities, and some of the most high-pressure moments (pennant races, playoffs, WS).
Not to mention Rivera has saved games at a nearly 90% clip for 15 years. Wagner is at 86% in almost 200 fewer chances. Hoffman's at 89%, barely behind Rivera, but again, didn't gain the reputation Rivera did with where he pitched and all the playoff experience.
Is Rivera among the greatest players ever? Probably not. Is there another closer I'd put in his league? MAYBE Hoffman, but Trevor's still a distant second.