Sure, IF Zambrano were to stay healthy and pitch a full year, he would almost surely be a better pitcher than Volstad, which makes sense for the Marlins "win now" mentality, but this move comes with a lot of risks. I guess that's what a team like the Marlins has to do at this point though when trying to compete with the Phillies.
Then again, I'm not a Marlins blogger, so I don't know this stuff as well as others would. Read this post from a real-life Miami blogger and that'll give you a good idea of what this deal really looks like.
This article is also a good read. If you scroll down awhile in the article you'll see a list of all the incidents and problems Zambrano has been involved with. It's a good humor-read.