Judges are human and subjective. They all come with baggage and can't be "impartial." Each has their own view of the Constitution and how it should be applied.
Rehnquist (Nixon), Scalia (Reagan) and Thomas (Bush 41) are the originalists or strict constructionists. Stevens (Ford), Souter (Bush 41), Ginsburg (Clinton), and Breyer (Clinton) are the judicial activists (read liberals). Kennedy (Reagan) and O'Connor (Reagan) are the swing voters.
As a result, the liberals control the court today and will still hold sway even if O'Connor and Rehnquist are replaced. The liberals can always count on four votes. They just need one other vote to control matters. A conservative replacing O'Connor would make it 4-4 with Kennedy being the deciding vote on most matters.
The Dutch could announce today that Natalie Hollaway had run off to Venezuela to wed Osama bin Laden, they are honeymooning in Yemen, and no one would care..