Too much size given up. When you consider 6'9 is a typical West PF and 6'6 is a typical West SG, the Spurs would be at a defensive disadvantage playing 2 undersized players in key positions at the same time.
Some West PF/SG bench guys I could think of, numbers might not be exact:
Lamar Odom - 6'10, Vujacic 6'7
Carl Landry - 6'9, Barry 6'7
Juwan Howard - 6'9, Rudy 6'6
Birdman 6'10, JR Smith 6'6
Milsap 6'8, Korver 6'7
There's practically no team I know of that plays both an undersized PF and SG at the same time.