1. For most of baseball history, three days of rest was the norm for starting pitchers, not some sign of super-duper-awesome-super-toughness.
2. If I had a horse like C.C. Sabathia or Cliff Lee in my rotation, you're damn right I would plan on using them in Games 1, 4, and 7 of an LCS or World Series. At a maximum, that schedule would go on for two rounds of the playoffs, i.e. three weeks, after which the pitcher is guaranteed a winter of rest.
3. Three days' rest isn't for everybody, and I certainly wouldn't force it on, say, today's Pedro Martinez. But would it really be that hard to go with your horse for Games 1, 4, and 7, then apportion the other starts for Games 2, 3, 5, and 6?