Rasho doesn't have nearly the quickness to chase Dirk around. Dirk would not have been an MVP candidate if teams could just throw slow 7 footers out there and shut him down.
With someone like Rasho, Dirk has enough speed to either create his own shot by moving to get separation or to create contact and get to the line. Even Tim would be hard pressed to stay out of foul trouble, which is one of the major reason he rarely covers Dirk.
The bottom line, as far as I can see: if the Spurs hadn't waited a quarter before showing up to the game, their plan for Game 3 would have worked perfectly.