It was one game. Scal played because if they had Shelden out there with Sheed, they were liable to see the Spurs redhead put up 30 points from the wide open 3's (I think he put up something close to that the last time they played). Wallace and Williams have both proven terrible so far this season of sticking with guys on the perimeter, and running them off 3's.
Ultimately, I think the decision should not have been between Shelden and Scal, but Shelden and Sheed. They needed Scal to stick with big Red (I am calling him that, because I am blanking on his name, and don't feel like looking it up...you know who I mean), So they had to choose between having Wallace getting destroyed by the much quicker (and harder working) Blair, while hoping he can make up for it by taking advantage of Blair's lack of size on the other end, or putting Shelden in to try to keep Blair a little more in check, but sacrificing even more on the C's offensive end.
Unfortunately, Doc went with Sheed, who kept forgetting that he had a midget covering him.
So what it really comes down to is...we missed Davis last night. He would have kept Blair more in check on defense, while giving the team a much better option on offense, who could have thrived inside against San Antonio's short bench.