Different strokes, I guess. I’ve always thought of the “It’s too late for me” exchange as being one of the most powerful in the entire saga, so I can’t wrap my head around the idea of thinking it’s useless. The whole point of Anakin’s tragedy and redemption is that he thought the dark side could not be rejected and could no longer see any good in himself. When Luke rejects the dark side and sees the good in Anakin, it inspires Anakin to finally break free of the dark side.
The PT shows us that Anakin was once a good man, but it doesn’t show us much of anything about Anakin’s mindset about himself after he becomes Vader. And while ESB establishes that Vader is reluctant to kill Luke, it doesn’t establish any deeper meaning behind it other than “he’s my son”. It’s the relationship they have in RotJ that develops it further, with Luke getting through to Anakin by choosing to see the good in him, refusing to kill him, and rejecting the dark side.