Given that Qui-Gon appears to have become The Fount of All Wisdom and Knowledge (despite the fact that he never showed any evidence of this when he was alive), surely he should be the default choice to advise and instruct Luke? At the very least you'd expect him to appear once or twice in the OT, and get the occasional mention, given that according to the Prequels it was him rather than Yoda who trained Obi-Wan.
Um... I always assumed that the Jedi did join the Force automatically when they died, and that this had been commonly known since the time of the earliest Jedi. Isn't it part of their Code, for heavens' sake? Why else would Obi-Wan tell Anakin that 'he is one with the Force' at Qui-Gon's funeral? I'm not saying that ghosts would appear very often, but the idea that it hadn't happened once before in over 25,000 years is insane - as is the idea that some random human Jedi, who didn't even come across as particularly 'wise' during his lifetime, would discover this ahead of someone like Yoda. You may be able to accept this; I most certainly can't.
And as for saying that it would destroy the purpose of their being wiped out, that's ridiculous. Ghosts can't touch things or fight battles or do anything except talk to people, and even that only happens on occasion. Did seeing (and hearing) Obi-Wan and Yoda as ghosts destroy the impact of their deaths? It certainly didn't appear to do so for Luke.