The thinking maybe that if Dyess starts, then the 2nd unit bigs would be Blair and Bonner--which would put too much pressure on a rookie 6'7" center to rebound (since Bonner is an outside 4)...whereas if Bonner starts, Duncan's size and rebounding could offset Bonner's poor rebounding, and Duncan's interior offense would benefit from the outside shooting of Bonner...and the 2nd unit bigs would have size and rebounding...
Or maybe Dyess is on the trading block? ;-)