I have a theory with regards to Willis going to Detroit -- I think Detroit only asked for Cabrerra, and the Marlins insisted the deal be expanded to include Willis. Willis is on the downturn, and the Marlins did not want to pay him. I think Detroit is taking a gamble on extending him, even at what appears to be a bargain rate.
Many commentators, especially the propeller heads at Baseball Prospectus, could not understand why Detroit was giving away top notch pitching prospects to take Willis. I think the Marlins drove the deal. I'm not a betting man, but I predict that Willis will be a bust with Detroit.