Supreme court justices are often active in the selection of new justices, advising the president as to whom he should pick.name and explain both....

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.