Of the 135 relievers who threw at least 50 innings:
Yes, you are reading that correctly. There were more negative value types than 2+ win types. Unless you are paying for a relief ace (and there doesn't appear to be too many of those), you may as well non-roster invite a bunch of promising arms and have a cheap and interchangeable bullpen.
Mariano Rivera was the lone 4+ W reliever. He had perhaps the best season of his closer career, which is saying a lot. He was worth 4.2 wins, equal to James Shields.