To argue thusly is to accept and enshrine bogus leftist cant. There is no precedent for any requirement that a nominee be ideologically similar to the departing justice, or that the existing "balance" of the court must be maintained. This is just leftist B.S. introduced at the present time because we have a President who has promised nominees more conservative than the current court. The "balance", such as it is, should result from the electorate electing successive Presidents based in part on their judicial philosophies. President Bush said what sort of jurists he would nominate, he was twice elected, so let's roll! Leftist propaganda should be scoffed at, rather than accomodated!
It's not bogus when our own elected R's cater to it. Whether or not I accept it, they do. As long as they're going to shrink away from "bad press," they should consider that when planning.
Trust me, Roberts will be the most conservative judge Bush puts on the court now that he has done this backwards.