And I don't exactly see how Miller makes it hard for SAR to post up unless Miller isn't hitting anything and his man plays off of him.
The reason Miller clogs the lane is that the concept only works in theory.
In reality, the center doesn't typically close out on Miller. Even when his shot is working. Miller needs to go 8-11 before people start to leave the lane to cover him (don't know why, but that's the way it works). The center stays home and plays safety. Which makes the post players contend with their man and the free-roaming center.
Further, when Miller is in his high post mode, and the center IS guarding him, it's easy to switch down and double from that position. The guy gets stuck in there and there is no where to kick the ball out to. Miller is the open man, but a fat lot of good it does because you can't swing it to him.
Whereas in a traditional set, where the center plays on the opposite side, the man who doubles usually comes from the perimeter. That means that if the man kicks the ball out of the double, a simple pass gets the ball to an open man for a deep open look.
If the double comes from the center side, then the man is left with two options: continuing the move and going up strong, kicking it out because the double would be weak, or if it's open - passing it for a dunk to the center. (KT/SAR do this quite often)
Miller playing outside does nothing for anyone except cutters. If you slice into the lane, there is a good chance the roaming center will pick off your defender and you'll get an opposite side layup. Plus it's easy to find cutters from that position. If the center is guarding Miller than it's even easier to cut in.
But, for pure post up ... the idea of a center who hits outside shots seems only to work in theory.