I think it was essential to show Vader's face at the end - to not have done that would have been a cop out and Sebastian Shaw's scene does bring closure to the story of Anakin. As you say, that was the whole point of ROTJ. And the notion that fans would never look at Vader quite the same again...well, again, that was the point.

I think it's a stretch to say that Kersh was saying the prequels shouldn't have been made, though. After all, he'd directed a film whose subtitle was Episode V - he must have known that Lucas would, at some point, make films beyond the original trilogy.