Another thing is wrong with this picture - force ghosts had never been established as living up to that "unimaginable power" Ben was boasting about before his death in the OT.
The maximum he accomplished was guiding Luke through the dogfight, which he could've done just as well alive, meditatin' or something, other than that his whole purpose was spouting off expository dialogue and joining in at celebrations.
No need to shine a giant spotlight at that (somewhat forgiveable) flaw from the OT, by shoehorning in a truly pointless scene with the sole purpose of fanservice and continuinty nod.
It's like "yay, Qui-Gon Jinn started this whole ghost thing! He teaches Yoda and Obi-Wan how to do that so in the next movies they can transform into ghosts and continue supplying our main hero with exposition! wait, why am I watching this?"
More powerful than any Sith? My ass is more powerful than any Sith. It all comes down to the width of the lightsaber, baby...