William H. Rehnquist was never a judge and he was a very good justice. But he had a background with the court (clerked for a Supreme Court Justice) and he was the Assistant Attorney General for the United States giving him the opportunity to deal the SCOTUS a great deal.
There have been Supreme Court Judges that were never judges before. I think it's a good thing as I said she's dealt with people and isn't likely to be an elitist. She knows what the average mans concerns are.