I hear this take a lot, and it is one I used to buy into. But eventually I thought about it more, and it just doesn't stand up. We SEE much of Luke's training. There is no emphasis made on combat of any kind. He is primarily taught how to control his emotions and resist the dark side. Luke wasn't trained to be a weapon, he was trained to be an impenetrable beacon of light. What Yoda was trying most of all to prevent is Luke BECOMING another Vader. That is a much greater threat than Luke merely failing to defeat him. Vader represents all of Luke's weaknesses, insecurities, and vulnerabilities. If Luke is able to stand against him, and stay true to the Jedi way - then he deserves to be a Jedi. Being powerful enough to kill him is completely irrelevant to his worthiness as a Jedi and is NOT his Jedi trial.