"We thought that Rogues had maybe felt a little bit not special"

Ghostcrawler said the above in an interview with IGN.

He talked about how the idea people have suggested of a "transforming" Legendary which might be used by a wider range of classes diminishes the concept, but oh this one acts like a sword if you're a Combat Rogue and a dagger if you're Assasination or Subtlely.

He talked about how ridiculous it is to expect Legendaries to be made for esoteric roles, but the only reason he could come up with for this design decision was that Rogues were feeling a bit sad and needed some cheering up.