That's a matter of debate.

You say Star Wars is about Vader... Well that may be how Lucas wants it to be now but back when the OT was made it was the journey of Luke Skywalker. Lucas once said that the whole saga was about the Skywalker family (he changed his mind since then like he always seems to), and IMHO it's better that way. We have the father's fall in episodes I through III and the son's rise in epidoses IV through VI.

That is why we see more of Luke than Vader : he IS the hero of the OT so the audience is following him, that's how it is in most movies. Vader is the bad guy so we only see of him what we need to see. You're right when you say we don't "need" to see transitions, but when not overdone they help us follow and get attached to the hero, which is Luke.