People saying Vader was fan service in RO: wasn’t he originally supposed to have all of Tarkin’s scenes before they gave the go-ahead for the CGI? I think he originally had a much larger role.
Plus, I think he fit just fine in the film. I actually think, unlike the ST, RO gets better with each viewing.
His appearance in itself wouldn’t be fan service if he did something of value to the plot. Mon Mothma and Jimmy Smits weren’t fan service. They had a tangible purpose in the film. I’d have much preferred Vader to the cartoon Tarkin though. His lightsaber battle actually might have been better had he been built up in the film itself and not just thrown in there under the assumption that he’s the most badassest villain in the galaxy.
You could argue that RO has a little bonus 4th act after the death of the new main characters to bridge the film to ANH. And the final Vader scene does add an extra obstacle at the very end when you think the battle is won. Sure, it’s hardly detrimental to the final film, but it does allow RO to end with a burst of action and suspense after the main story has been wrapped up. SW movies can’t just be movies anymore, there’s too much lore. If the escape of the Tantive IV had been omitted I’m sure people would have complained about that too.