Volstad just didn't perform. That was a terrible bit of drafting that year by us - 5 first-rounders, all spent on pitchers, and Volstad is the "best" of the bunch.
This has a chance to work out well for us for a year. Zambrano is in his walk year (unless he finishes in the top 4 in the Cy Young voting, in which case his 2013 vesting option kicks in at $19.25M). Maybe that will motivate him to behave. Ozzie may be able to control him for a year. Even last year, in his worst major league season, his numbers were comparable to Volstad's. Before that he was clearly much better than Volstad.
This also has a chance to blow up on us. Zambrano's not old but he's been around 10 years so he could start breaking down physically. Between Hanley pouting, Zambrano's mental health issues, and Ozzie's mouth, this could be a huge mess. But at least it's only for a year.
We didn't give up anything either. The Cubs are paying the difference between Zambrano's salary and what Volstad will probably make in arbitration. Volstad has been mediocre at best - the only thing he has going for him is that he's pretty young (25) and pitchers sometimes figure it out late.
Zambrano can hit too. He has 23 career HRs and a .241 career batting average. I bet we'll see him pinch-hit several times this year.