The way I view it, the chances of Miller being the difference between winning a world series and not winning one is miniscule. He's one player on the 25 man roster, and plays very little. Was it worth giving up a top 100 prospect who was close to being ML ready for a 1-2% better chance at winning the world series? I don't think so, though I guess some people might. I don't like going all in for these kinds of deals unless you get a major impact player who can actually make a difference, like an Ace or a middle of the lineup star.
