Yeah my gut feeling is that the relative balance is important to people, but I understand that if one party wins 2 or 3 elections in a row and there happens to be a lot of vacancies, they could swing the court heavily to the right or to the left. And since game theory dictates that you don't know when you'll have the presidency again, you can't nominate centrists to please the crowd you may find the court packed with your opponents + centrists...which means it leans to your opponents.