I think it depends on Wade. Although it's certainly possible for Miami to beat the Pistons without him, if he doesn't play, the Heat will have a much tougher time.
I'm confident Miami will win the series, but it's probably in their best interests to play an extra game against the Pistons before playing the Spurs. The Spurs are already getting some rest, so giving them a few more days isn't going to make any difference. But the Heat will probably get at leat two days rest after beating the Pistons anyway. And, the longer they're playing the Pistons, the longer the Heat can wait for Wade and Shaq to get healthy.