You're right, the Parker/Manu/Bowen/Duncan/KT+Oberto lineup has no defense at all and has failed the Spurs offensively and defensively in the past. And of course, Carter is going to do a bang-up job guarding bigs.
Carter as a defender in my mind does not take pressure off the Spurs front court. He may be a decent wing defender, but I fail to see how my original point does not stand: if the Spurs are seriously seeking to bring in VC this may signal their decision to outscore, not out-defend the bigger (ie Lakers) WC teams.
Edit: Put Finley in any Spurs lineup and it automatically becomes the Spurs worst defensive lineup.