The Suns have no chance in Game 5. They'll start Thomas/Marion/Bell/Barbosa/Nash and that's it, that's the whole team, for 48 minutes. Jones and Banks will get a little time but will be useless. Through hustle and determination, the Suns might keep the game close for two or even three quarters, but there's no way they can sustain the energy they need to in order to be competitive for the whole game.
But the worst part is, they will wear themselves out trying. That's the hidden cost. They'll leave everything on the floor for Game 5, for as long as they can, and they won't have anything left for Game 6. As we saw last night, they can win in San Antonio, but only if everyone is able to play hard the whole game. (As it turns out, the Suns did not play particularly well last night, but they did play hard.)
The bottom line is that the Suns have to win Game 6, and they have to win Game 7. Now, what gives them the best chance of doing that? Being as rested and as focused as possible.
Game 5 can only harm them. It will wear them out, distract them, expose them to further injury, and give the Spurs even more information about how they play. Nothing good can come out of it.
The best strategy for Game 5, then, is clear: Don't play it. Accept being down 3-2, and get ready for Game 6.