It does defeat the purpose, doesn't it? If the writers feel like bringing characters back is a means to shake things up, then I don't think they have a firm grasp of the universe they're operating in. Personally, I think that they just have a story they want to tell with the character but didn't have the chance because it was killed off before their turn. Maybe they don't have enough confidence or creativity to have the legacy characters pick up where the deceased characters left off. It's a shame really.