Mandamus Mystery?

A recent mandamus opinion, In re Sealed Petitioner, made this provocative statement about the potential role of a mandamus writ:

But of course, “the federal courts established pursuant to Article III of the Constitution do not render advisory opinions ….” E.g., Pub. Workers v. Mitchell, 330 U.S. 75, 89 (1947). Fortunately, review of the cited case shows that it just states principles about mandamus review that are generally accepted as part of today’s standard practice (citations omitted):

Although the Company correctly observes that mandamus has historically been a drastic remedy generally reserved for really “extraordinary” cases, the federal courts of appeals (as well as the Supreme Court) have shown an increasing willingness in recent years to use the writ as a one-time-only device to “settle new and important problems” that might have otherwise evaded expeditious review. As the District of Columbia Circuit explained … “Schlagenhauf authorizes departure from the final judgment rule when the appellate court is convinced that resolution of an important, undecided issue will forestall future error in trial courts, eliminate uncertainty and add importantly to the efficient administration of justice.”