This would make more sense if there was a great young catcher waiting in the wings for Detroit, but that's not the case. Since the Pudge trade, they've been going mostly with Brandon Inge, a light-hitting career third baseman who'd already been slated as the likely 2009 catcher, and longtime minor leaguer Dane Sardinha.
It's possible that the Tigers front office considered it worth sacrificing some offense, which they have in abundance, for some help with pitching which, again, has been a weakness all season. Even so, the move has weakened the Tigers' defense: the Rays challenged them on the basepaths during a weekend sweep, going 3 for 5 in stolen bases on Friday and 2 for 3 in on Saturday - aggressive even by Tampa's standards.
The Tigers' decision to trade Pudge seems to lie in Detroit's offseason blockbuster deal for Cabrera and Willis which improved their lineup (if not their rotation) for this year, but, along with earlier trades, drained the organization of prospects. Needing bullpen help, the Tigers apparently had no choice but to give a present contributor. So far the risk hasn't paid off, as Detroit has lost four straight and has fallen even further behind Chicago and Minnesta in the standings.