- Supreme Court justices are legally qualified. Although there are numerous lawyers serving in Congress, the primary role is political, not judicial. Serving judges are the best people to decide who would be the right candidates for the bench.
- Getting Supreme Court justices involved takes some of the politics out of the nomination process. What ultimately matters is not whether justices are conservative or liberal, but rather their ability to decide complex cases.
- Nominations for the Supreme Court are a long-standing part of the democratic process. This is one of the few ways that the people's representatives can exercise any degree of control over what is an unelected institution. Greater involvement by judges in that process will further serve to perpetuate the existence of a judicial elite, far removed from the public's concerns.
- It is naive to suggest that the Supreme Court is purely a judicial institution. It also has a very important political role to play in the American system of government. Supreme Court decisions are, of their very nature, deeply political, relating to contentious issues such as abortion and civil rights. Serving justices can certainly be sounded out for their opinions, but their role in the selection of new justices must be a subordinate one in what is primarily a political process.
The process of nominating a new Supreme Court Justice can be rife with political bias, often resulting in the President or Congressional leaders lobbying for justices who typically support their own political party. In consulting with sitting justices, the President is offered a perspective no other humans can offer—that of an experienced confrere.
The key benefit to including sitting justices in the nomination process is the advantage of real-world experience. Since no one can truly understand the responsibility of acting as a justice if one has not done so, sitting justices are able to offer suggestions for experience, personality traits, and abilities a great justice should have.
A possible drawback to relying on the advice of sitting justices would be their likelihood to hesitate to stray from the status quo. While experience and norms are important in every branch of government, so too are new ideas and varying perspectives. In upholding the Constitution, it is beneficial to create a court that is able to see things from differing points of view.
An advantage is that the Supreme Court justices are the best legal scholars in the country, have held the job a long time (the average term is 26 years) and have a lot of experience with Constitutional matters. So having their advice about what new nominee is capable can be an advantage. If a President is looking for a liberal or conservative justice to take the open seat, they can get advice from those in the job already too.
I do think "cloning" the Supreme Court is a bad idea, but some consistency in judicial rulings and a respect for established law is a good thing. As the balance on the court remains roughly the same over time, we get consistent rulings on established law. It's a good thing that the interpretation of the founding document doesn't swing back and forth on a regular basis. As the President can't remove them from the bench once they are confirmed, it can be a bad thing if a Supreme Court justice has too much influence over a new choice.
The most obvious positive benefit of this would be that the justices who are already on the Court would be more likely to respect and to be able to work with the new nominee.
A second positive impact would be that Supreme Court nominees would probably be picked more on legal merit than on politics if the justices themselves had more of a say in the process.
The major negative would be that the Court would sort of tend to clone itself. There would be less likelihood of new blood (ideologically speaking) coming onto the Court.