Rafael Furcal as he's performing right now is an upgrade for nobody. Saddled with a sub-.600 OPS and playing, according to some observers, the worst defense of his career, Furcal is, based on his 2011 numbers, a kind of terrible cross between last year's Brendan Ryan and this year's Ryan Theriot. But with Theriot performing just as poorly, the Cardinals' apparent trade with the Los Angeles Dodgers has given their offense, downgraded by their recent trade of Colby Rasmus for starter Edwin Jackson, a little more room to breathe in August and September.
If Theriot and Furcal are playing identically right now, their careers to date have been drastically different. Theriot's tools--hitting for average sometimes, taking walks sometimes--are intermittently visible and never especially easy to see over his deficiencies on offense and defense. His poor play in 2011 is at the lower bound of reasonable expectations for the year, but not exactly stunning.
Furcal's poor play is unprecedented, though his fragility is not. By acquiring him the Cardinals are paying a significant (but neither unprecedented nor exactly stunning) amount of money for the chance that they might end up with the real Rafael Furcal as their infield upgrade heading into the playoffs, instead of the real Ryan Theriot.
If he's done, it's no harm; the Cardinals aren't blocking anyone except Daniel Descalso, who'll get plenty of playing time anyway, and Tyler Greene, who wouldn't get plenty of playing time anyway. If he's not done, the Cardinals have a real shortstop--for far less than it cost them to get a real starter.