I think six movies about darth vader was enough. His story ended in return of the jedi. Bringing him back again would cheapen him.
I don't buy that for a second. Vader is the biggest icon of Star Wars and the ultimate scifi villain. He and the droids are the only characters they need to bring back.
Besides, if Hayden returns as Anakin's Force ghost then technically we'll still see Darth Vader in the new movies sans the black suit. And the Dark Side.
I get the feeling that since George said the SW Saga was really the story of Anakin more than anybody else that we'll continue to see him in spirit form giving advice and wisdom to his son and daughter as they try to live and forge ahead in the post-Empire galaxy.
Anakin Skywalker doesn't need to come back, Vader does. I don't care if we never even find out who is inside the suit. Bring back Vader and make him genuinely menacing.