The Tigers trading Nate Robertson to the Florida Marlins makes sense on a number of different levels.
One is they might be saving money, although how much is unclear at this point. It's doubtful the Marlins are picking up anywhere near all of Robertson's $10 million salary for the coming season. Also, Jay Voss, the pitcher acquired from the Marlins, is being developed as a left-handed situational reliever. He isn't that far from the major leagues, having done well in Double-A ball last season (3-0 record, strikeout per inning, not that many walks, opposing hitters batting just .200 against him).
He has a good arm, touching in the low 90s with his fastball, and more importantly he throws a couple different versions of a slider, which makes him more effective against left-handed batters. It allows the Tigers to begin the season with Dontrelle Willis in the rotation, but also for a couple backup plans in the bullpen should there be a need to move left-hander Phil Coke into the starting rotation.
One thing is apparent: Robertson isn't effective out of the bullpen. He also had some trade value. Willis has none.
The Tigers still would have plenty of lefty depth if Coke is moved into the rotation with Fu-Te Ni, Daniel Schlereth and Voss - despite the injury status of Bobby Seay.

Transcript of my live chat today at theoaklandpress.com: http://tinyurl.com/yl3bge5