Dumb move. The Suns are a dangerous team, and sitting Manu, arguably the Spurs playoff MVP, would be a huge mistake; if for no other reason than it would piss the Suns off and give them the extra motivation to win a game and extend the series.
In one of the post game interviews, a reporter asked Tim what the Spurs were doing to win. One of the things he listed was respecting the Suns and their ability. Sitting Manu when he's not seriously injured to save him for the finals would be so blatantly disrespectful and arrogant that the Suns would most likely come out on fire. The Suns have been put in a position where they've had to come from behind after the first quarter all series. If they come out of the first with a lead, it changes the whole dynamic of the game; perhaps enough for the Suns to win. Why risk going back to Phoenix for a game 5? Because then Manu will have to play. The Spurs lose game five and it is now 3-2 with the Spurs in a must-win situation in game six in San Antonio; because you don't want to go to a seventh game in AZ. Every extra game this series goes reduces the rest the team as a whole can get. (Duncan needs a week off to give his ankles time to get close to 100%. No matter who comes out of the east, Duncan will have a harder task before him playing against a physical, championship tested frontcourt than what he's faced against the Suns.)
If the Spurs win, they'll have over a week off before the finals. Pleanty of time for El Contusion to heal up. You have to play to win.