If Luke wins, the Emperor gets a shiny new apprentice.
I don't believe that one bit, actually. There's no reason to believe Luke would ever join the emperor, even if he killed his father, Darth Vader (lol), using the dark side of the force. I'd reckon Luke would rather be killed and tortured by the emperor than join him. Throughout the entire tirlogy there is no indication of Luke descending to the dark side, with perhaps the exception of the philosophic cave meeting that took place in ESB and maybe some of the anti-Obi-wan-ness. If George Lucas wanted us to believe Luke could turn to the dark side, he should have made a better case imo. Still, this is only a very, very minor flaw for me.