Thomas has his issues and he'll never be a star, but he's essentially a young McDyess (not as good of a scorer as McDyess was though). It's clear from their comments that that's what they'd like at power forward. There aren't many players in the league with this type of package and most of the other ones aren't attainable, given what the Spurs have to offer.
Blair and Anderson are solid young assets, but let's face it, at their peaks, Blair will probably be no better than a 3rd big/7th man and Anderson will probably be no better than a 3rd wing/8th man. Plus, if they added Thomas, Blair wouldn't play anyway.
TJastal, they may not. A former 1st rounder and early 2nd rounder; that's irrelevant. Thomas is a former 4th overall pick. Once a player is a couple of years removed from being drafted, where they were drafted no longer matters. Does anyone think Brown and Milicic have high trade value, because they were former 1st and 2nd overall picks? No.