It looks like Chris Volstad has the last rotation spot locked up. If you looked up "serviceable" in a dictionary there would probably be a picture of Volstad next to it. He's been in the bigs for most of four seasons, and made 87 starts in his last three years with the Marlins. Perhaps one of the reasons that Thoyer targeted him in the Z dump was that his peripherals looked pretty good last year (3.64 xFIP) despite a 4.89 ERA. He doesn't strike a ton of guys out, but that's not too surprising given his strong ground ball tendencies. Weirdly enough despite being a groundball pitcher who pitched half his games in the cavernous Sponsor Of The Week Stadium in Miami, he has given up a ton of HRs. The Cubs are betting on that normalizing.
Here's his 2012 projections
This projection, based on the run environment in the past two years, comes out to 9.72 RAR, or 1.0 WAR. Given my grumbling about FIP undervaluing groundball pitchers, he's probably worth a bit more but we have to use a common framework to compare everyone. There's not a lot of upside here but he's cheap and will give innings, just what the Cubs need for now. If the Cubs had simply signed him or something rather than traded Z for him Randy Wells would probably have his spot, but them's the breaks.
Previous pitcher projections