I'd like Diaw's defense in the starting lineup -- I really would. But there's just absolutely no way that Blair and Splitter will work as the bench bigs. Splitter has to have a floor spacing power forward next to him. It's a must. If the Spurs try to play Blair next to Splitter, the opposing team will just sag off of Blair and negate all the Splitter pick-and-roll attempts. And it'd be a disaster for Blair, too. Not only would he have to try to score over his man (who is always taller than him), he'd have the opposing center also in his lap.
On paper, Blair and Splitter doesn't work at all. The stats back that up about as emphatically as statistically possible: Last year, the Spurs outscored opponents by 15.31 points per 100 possessions when Bonner played next to Splitter. When Blair played next to Splitter, the Spurs were outscored by 13.11 points per 100 possessions. That's a staggering difference of 28.42 points per 100 possessions when you put a floor spacing next to Splitter as opposed to an interior player.
If Diaw starts, it GUARANTEES Bonner will be the backup power forward. Blair next to Splitter is an impossibility.
As the roster is composed at this moment, starting Blair is the lesser of all evils. It's obviously far from perfect ... but them's the cards that have been dealt.