I'm not even sure what you are even getting at. They are using the best pitchers on the team in low leverage situations. That cannot be argued.
Cleveland could come in and blow the Cubs out tomorrow, but that doesnt mean it was the right move. Why ride guys when you don't have to?
You said it yourself - they may not NEED him tomorrow.
Francona made a great call IMO. He seized the chance to absolutely take the series by the throat. They now only have to win one of three - AND have a day off. If he'd like, he can make Miller unavailable today and still have him for multiple innings in both Games 6 and 7. If everything goes wrong Sunday and Tuesday, the Cubs will still be staring at Kluber and Miller one more time - and in Cleveland to boot.
No one who ever complained about Fredi's "unbelievable faux pas" when he didn't go to Kimbrel can possibly be serious when criticizing this move. This is the World Series, and it's all hands on deck - if Miller was tired, he'd have told Francona he needed a day.