No one on the staff won more than a single game more than Zambrano did. And he had an ERA about equal to Dempster, who will be on the team next year.
Pitching wins are meaningless. All you have to do is look at Matt Garza's win total compared to anyone else on this team, and that becomes clear.
All Dempster's peripherals (K%, BB%, HR%) were right in line with where he had been the last several years. According to Fangraphs, his fielding independent stats (FIP and xFIP) were virtually identical to 2009 and 2010, and just a little worse than 2008. After his awful April, the results matched the peripherals too. He threw 200 innings. He was about the same pitcher he has been since 2009, but with terrible luck for the first month of the season.
In contrast, Zambrano's K% cratered. His HR% skyrocketed. He hasn't thrown 200 innings in a season since 2007, so I think it's safe to say his days as a real innings eater are over. To his credit, his BB% dropped. And his FIP and xFIP both showed him to be a little unlucky (though not nearly as unlucky as Dempster). But there were definitely some warning signs in his peripherals that he should have struggled.
There's no upside to Zambrano anymore. He's a lame duck mid-rotation starter if everything goes right, but more likely a back end guy who may erupt at any time. You can find several back end starters on the market for $5 million or so who haven't been suspended by their own team 2 years in a row. It's time to move on. And I'm saying this as someone who has been one of the few consistent Zambrano supporters on this board for the last 3-4 years.