I think that line in Empire is incredibly good, and in retrospect it's really quite obvious that Vader really is Luke's father, and that Luke knows it. But back when those movies came out, it wasn't seen that way. Just as the "other" Yoda speaks of to Ben is clearly Leia, retrospectively, because she senses Luke calling for her and she rescues him. But again, the debate in fan magazines raged on for three years about those two questions.

So why remove them, just because you know the answers now?