Ok, I'm calling this one, we only have guests still because it's one of the things that makes SC more then just another fighting game... Why buy SC with a load of old style characters you've never heard of/don't care about when you could buy Tekken or SSF4.
And in my opinion it makes the game better for it, technically, the guests have never been faliures, well, I never played BD, so I'm going to pretend Kratos never happened for the purposes of this (they've been integrated into the actual gameplay well), and while I'm sure many round here might disagree, from a technical standpoint SC3 was... They add something to the gameplay that can't be accomplished with just the barebones weapon fighting standard we all expect.
As far as Vadar goes, the movies never did that well on making him much more then a lumbering giant and a symbol of power through fear, it wasn't until the new movies that the lightsaber combat gathered an identity of it's own, as something more then fencing with lasers. Making him look like Vader was the easy part...
And also, what is it about stabbing, slicing and gutting your opponent, repeatedly round after round (which has been done since Soul Blade, and before guests), that establishes continuity? Making things a little more mysterious adds to the enjoyment and appreciation of the work that's gone into writing the story then making the game.
