I think her point is simply that, as shown with past 'unknown' appointments, one has been surprised at how they rule once on the court, Souter being the obvious example. When these appointments are so rare, it is safer to go with a known item, such as Scalia. I have been mighty impressed with Roberts through the hearings last week, but who knows what he'll actually do once he is on the court.
Her position has been duly noted. We'll just have to wait and see how Roberts presides and rules, and who President Bush nominates next.