Darth Vader did it. Then the Green Goblin. Now even Sauron has got in on the act. It’s one of the oldest tropes (or should that be clichés?) in the writers’ playbook: the bad guy inviting the hero to team up.
There are dozens of examples, and most of the time it’s entirely pointless. Nobody in the audience expects Spidey or Galadriel to suddenly abandon their principles, so the story spins its wheels to no purpose and then moves on.
Those writers might think they’re paying homage to The Empire Strikes Back. After all, we don’t really expect Luke to turn evil either, do we, even though he has been tempted by the Dark Side? But that’s not what is really going on in this scene. The emotional impact is that we finally see what Vader wants – to have his son at his side. More than that, he genuinely believes that together they could rule well.
Most importantly, Vader’s offer sets up the denouement of Return of the Jedi. Luke doesn’t join Vader to overthrow the Emperor, instead it’s the other way round, completing the moral and emotional arc begun in that showdown on the gantry in the previous movie. Finally Vader realizes what really matters to him.
That’s why that scene is astonishingly powerful and has never been copied successfully. Accept no substitutes.