I'm puzzled, too, but I suspect there is more going on than meets the eye.
As Counsel, Miers has worked with the President. Bush has gotten to know her, to know her views. They are certainly not very different from his own.
Miers has no judicial record, and a thin record was the one bit of damp ammo the Democrats had on Roberts. That and the 'Texas Mafia' line (Miers and Laura Bush were classmates, I've read) may be all they have on Miers as well. We'll find out soon enough. No record and a charge of cronyism may be enough to sink the nomination. In which case, one might assume that Bush did the political calculus, and has another nominee in the wings.
Bush isn't running for re-election. His poll numbers are pretty much meaningless. But with cries of dismay coming from the right, the opposition simply isn't going to be energized, except for the terminal-BDS crew.
Crazy like a fox? I hope so.