Look at the Brewers last year. IMO, the success they had is heavily attributable to the 8th and 9th inning relievers they ran out there in almost every winning game.
It's not as if the major leagues are overrun with great relievers so that each individual is of no importance. Many teams, like the Reds, have struggled to put together competent bullpens.
I can understanding discounting the value of middle relievers who may be interchangeable. But late inning types, with power stuff, are just as important as any other position on a team, with the exception of top-of-the-rotation starting pitchers.