I don't think a nominee's personal beliefs matter at all. What matters is what the constitution says. What is important is that any nominee understands that. Beliefs change, character does not, so character is very important. If you believe that the constitution says what it says then that's exactly what we want on the court. All other issues follow that reasoning.
Well I don't think I disagree with that, and I'm sure W is judging her on her professional demeanor. But we all know that as soon as a person gets onto a place like the Supreme Court where no one can force you to be accountable to anybody, there's a big temptation to let your whims have rein. Blackmun, Kennedy, Souter, and others all started out as reasonably conservative in their first year, just as they appeared to others before confirmation, but with no one to answer to they all soon enough discovered the bleeding heart that was always within them.
The danger in a nominee like Miers is that there's no evidence she's thought deeply about why she's a conservative, what it means to be a conservative, or what any of that means when it comes to interpretation of the constitution. With someone like her, there's a big danger that her personal beliefs are all that matter. There is no reason to think she has any objective philosophy of jurisprudence to fall back on.