Rather than relying on a single test like outcome-determinative, there should be a general balancing of interests. In federal diversity actions, the federal procedure displaces state provisions within its scope. Both plaintiff and defendant take discovery, both sides have privacy and confidentiality concerns. Uniform federal discovery rules are necessary because federal courts have limited opportunity to review discovery rules on appeal. While the Supremacy Clause provides the right of federal courts to apply federal law when in conflict with state law, courts are reluctant to do so if there is no compelling federal interest or where state courts have historically had autonomy.
States are laboratories for legal innovation.
Constitution gives Congress authority to establish federal court system and to take necessary and proper measures to effect that authority. Given national interest in uniform federal court system, the fact of conflict between federal and state rules for service doesn't mean the federal government retreats, Hanna. Whenever possible, uniformity should be pursued in order to reduce confusion among citizens, and disparity in judicial decisions in like cases. In Re Agent Orange. Where only private litigants' rights at issue, there's no reason to apply federal law even though a federal agency is a party to a contract dispute, Miree.