In "Darth Bane Path of Destruction" one of the instructors said
"The Darth title was more than just a symbol of power; it was a claim of supremacy. It was used by those Dark Lords who have sought to enforce their will on the other Masters. It was a challenge — a warning to bow down or be destroyed."
So if the title Darth is reserved only for those strong enough to claim supremacy why did the Emperor just grant his apprentice this title? It seems completely out of character, for him and Sith in general, to grant his apprentice such an undeserved elevation.