It was not a matter of supporting Miers that upset most in the pro-Miers crowd, but that "the process" was Borked.
Confirmation would result in Ms. Miers sitting on the court for the next 20 years. Given what was learned about Miers over the last several weeks is that a scenario you're comfortable with?
Rejection by the Senate would have been far more politically damaging to the President than withdrawl. Republicans voting down a Republican President's nominee? That sounds just super, especially to the rabid left.