Comprehensive knowledge of laws is important. Much of our legal system was built on English Common Law. Foreign legal theorists such as Montaigne played an important role in the development of ours.
This is a non issue unless you are willing to exaggerate any impact and distort the realities of judicial research.
All of which is totally irrelevant to the whether current trends in international law should play a role in interpreting the U.S Constitution as espoused by Stevens,Ginsburg and O'Connor. Fundamentally, it is an issue of national sovereignty.