I'm not saying he's going to improve as a pitcher in 2008. I'm saying Belisle can be the exact same quality of pitcher in 2008 as he was in 2007 and we would expect his ERA to drop by nearly half a run. Because pitchers that do the things Belisle does have ERAs in the mid to high 4's, not the low to mid 5's. This is the key take away. It's that he's going to magically become a better pitcher. It's that he doesn't have to in order to justify a rotation spot. Your criticisms of him as a pitcher are spot on. But that's why he's a pitcher with a 4.64 career FIP. He's mediocre. That his ERA was over 5.00 was a result of a 4.64 Matt Belisle with half a run of bad luck and bad defense stirred in.
Yes, a top pitcher can succeed in spite of his defense. Frankly, I'd love to have a staff of 5 Aaron Harangs. But looking around free agency and the trade market, and given that Belisle is currently our #3 starter, there's no way in heck that we can obtain 3 pitchers who are better than him.
Lohse, Silva, Lieber -- all not better than Belisle. Maybe about the same, but not worth spending resources on to displace Belisle. Again, I don't like Belisle as a #3, but he's not Eric Milton either. He's a solid #4 starter on a good team. He's Jeff Suppan. You don't want to build your rotation around him, but he fills a spot capably and better than you can get without major costs.
Fill the other two spots in the rotation with guys better than Belisle, then we'll see how much of a priority it is to find a replacement for him.