Is it because we're paying him too much?

Heilman has performed to the tune of a 4.47 ERA and a 1.55 WHIP, both worse than Carlos Marmol and Kevin Gregg, who haven't had the best of seasons.

He has appeared in the 3rd most games of any reliever, and has thrown the 2nd most innings of any reliever.  Those appearances should be held for Angel Guzman, who is our best RP so far this year.

While they have small sample sizes, both Jeff Stevens and Mitch Atkins can't do much worse.  Stevens has a 2.18 ERA and a 1.09 WHIP in 40 games in AAA this year, while Atkins hasn't performed well at all in Iowa, he did throw 2 solid innings for the Cubs this year. 

Teams all across the MLB seem to udnerstand the idea that young talent is a lot cheaper.  In this case, it's most likely better too.