|Via Wikimedia Commons|
That's undoubtedly true, but courts, including the Supreme Court, cite the Magna Carta as the original authority on a variety of important legal issues:
-8th Amendment prohibitions against cruel and unusual punishment, Solem v. Helm, 463 U.S. 277, 284 (1983)
-due process, Strahler v. St. Luke's Hosp., 706 S.E.2d 7, 15 (Mo. 1986)
-open courts/access to the courts, Craftsman Builder's Supply, Inc. v. Butler Mfg. Co., 974 P.2d 1194 (Utah 1999)
-law provides a remedy for injuries to person and property, Smothers v. Gresham Transfer, Inc., 23 P.3d 333, 340-41 (Or. 2001)
-subrogation rights, Liberty Northwest Ins. Corp. v. Oregon Ins. Guarantee Ass'n, 136 P.3d 49, 57 (Or. Ct. App. 2006)
So, maybe the moral of the story is: the Magna Carta is good authority, as long as it's not your only authority.