The thing to keep in mind about tkeeping he "Darth" thing is that the whole defence of telling Luke that his father is dead is that to Obi-Wan, he DID die when he became Darth Vader - for him, Anakin is no more, and, incidentally, it's the explanation of why Luke is able to succeed in rescuing his father where Obi-Wan failed; Luke still believes his father is in there somewhere. Unless you were kidding, Eros?