The reason he was in such a short cameo was because, what with Peter Cushing being dead and all, he was completely CG, which would have looked weird having him make more of an appearance than he did. Sure, they could have got another actor to play his role, but you have to admit, the man had a very distinguished face that would not have easily been substituted.

Ultimately, the whole Tarkan issue is another problem that was created thanks to the amazing changing story line of the Star Wars Saga. Originally Vader role in the grand picture of the Galactic Empire was quite different from where it wound up. Yeah, the prequels absolutely could have and probably should have retconned a reason for the second most powerful man in the galaxy being ordered around and talked down to by someone the same rank as the guys Vader uses as whipping boys in later films.

But after seeing the Phantom Menace, did you really think there was even the slightest chance this would have been explained? The prequels followed a strict rule of explaining crap that didn't need explained and not even coming close to approaching things that people have been curious about for nearly 30 years. Remember 3PO's mismatched leg? That is one me and my friends always talked and wondered about when we were kids, not sure why it fascinated us so much, but we knew there had to be some story behind his silver leg. Before 1999, I was pretty sure this would be explained in the PT, after 1999 I was pretty sure it wouldn't be touched on at all. PT did a brilliant job of showing us all sorts of crap we really didn't want to see, and not showing us anything we really did want to see.