Doubt means to be uncertain or distrust something. We have reason to be uncertain or distrust the Emperor in that scene after viewing ROTJ, TPM, AOTC, and ROTS. It doesn't work perfectly once you watch four other movies of the Emperor plotting and scheming and betraying his servants. Specifically on the topic of a new apprentice. There becomes doubt at the idea that the character of the Emperor needed to be convinced with something as simple as "he would become a powerful ally." If anyone in that scene knows about using powerful allies and exchanging one apprentice for the other, it's Darth Sidious. Thus it becomes less believable that he didn't consider for one second the possible uses of Luke Skywalker or that if he did and dismissed it, someone saying something as simple and as obvious as "he would become a powerful ally" would suddenly change his mind. As for the other things you brought up... 1) Anakin was nine or ten years old when the Emperor first took notice of him. Far too young to help him at that time (not like the clones were ready anyways) but something to use further down the road. 2) When Anakin hesitated to kill Dooku, he still has two lightsabers at Dooku's throat. Quite different from Luke throwing his lightsaber away. 3) When it comes to turning someone to the dark side, it comes down to that moment where the person gives into their fear, anger and hatred etc. The Emperor didn't spend 10 years turning Dooku but he passed the dark side test. Luke however did not. 4) I am not arguing that Sidious did not see value in Vader once he was burnt up. He saw a man tortured and in pain who needed the Emperor more than ever. His hate keeping him alive. Plus it's not like there were many force trained people hanging around ready to step into the role as right hand man of the Emperor at the start of the Empire. Vader: "What of the reports of the Rebel fleet massing near Sullust?" Emperor: "It is of no concern. Soon the rebellion will be crushed and young Skywalker will be one of us." Turning Luke was just as much of a priority to the Emperor as it was crushing the rebellion. It's enough effort in regards to the films when he tries to bring Luke to him in the Bespin trap and it fails, so he tries again in the next movie. Darth Sidious betrayed and murdered Darth Plagueis. The possibility of any apprentice betraying him is always on the table. Two of those things he said while he was trying to draw the anger and hatred out of Luke and the other was said when the Emperor was angry and in the middle of killing him. That said, I don't think the Emperor thinks highly of the intelligence of anyone he is manipulating or plans to manipulate. He just cares about what they can do for him.