The key difference:
This year's payroll was approved with the belief that the current roster had a realistic shot of competing for a championship.
Adding playing like Thomas and Salmons, and perhaps Splitter, to the existing core make the team better, but even through rose-colored glasses it is difficult to view the Spurs as championship contenders.
I've reached the point where I don't believe that there are transactions out there that make the Spurs into contenders again in the Duncan era, even if Holt were willing to spend like Cuban. I don't think that they'll follow the Portland or OKC example and blow the whole thing up, but it make sense to let the older pieces leave by trade or contract expiration and replace them, one by one, with younger, more athletic players.
Then you proceed with an orderly transition towards the post-Duncan era. They tried to keep the championship open with the notion that the inevitable rebuilding could wait until 2011 or 2012. If championships are no longer possible, there's no reason not to begin that process right now.
I still think that if they can get Thomas/Salmons for Finley/Mason/Bonner then they should, but they may have reached the point where role players on the wrong side of 30 are no longer interesting for MLE level salaries, even for one year.