Hm, as old as the debate was, would've been cool to see him back for that. Though we wouldn't have 'seen' him either way. Vader's suited role was so minute in III, so it really didn't matter. Might have been counterproductive to get him back and add another name to the pay roll and training, not that there would be much, but Hayden's been there for the past six years or so, and Prowse has been on a leave of absence for awhile.