Well, yes, it's certainly not a character study, but if you establish the precedent that a character can make a choice between good and evil, and that they are not simply stuck on one side of the line or the other, it implies that the characters themselves are not inherently one or the other. You say Vader is non-relativistically evil, but Luke constantly says, "There is good in him." And it turns out he is right, which means that Vader has both good and evil traits. True, there is not really any good example to show this. We don't see any struggle until the very end, so I'm not arguing that it did a good or deep job from a relativist angle, but the concepts certainly existed.