The truth behind Jedi vanishing at death!
George Lucas was originally going to have Ben "return" in episode 6 shortly before Luke confronted Darth Vader and Darth Sidious/Palpatine. Ben was ghost to appear before Luke, then his ghost form would become "flesh and blood" again. Ben (and Yoda's ghost) was to help Luke battle Darth Sidious during the Force Lightning attack. At the end of the movie Yoda and Anakin Skywalker were to appear as ghosts (which they did), then regain their flesh again. This was in the orignal script of episode 6. The Return Of the Jedi script was rewritten several times as George Lucas threw ideas he didn't like and replaced them with better ones (as he often does). This idea of Jedi returning after being killed is too farfetched. Not that I don't like it, it just takes away too much. George wanted to make the confrontation much more dramatic. He did so completely by dumping the idea of Ben returning "in the flesh" and later dumping the idea of Ben and Yoda's ghost's jumping into the confrontation. It made Luke character more independent and powerful. He triumphed by himself (okay, with the help of the "returned" Anakin Skywalker) and the Sith were totally defeated. Master Qui-Gon Jinn didn't vanish because . . . it wasn't in the script (literally)! Now, if Master Jinn will return as a ghost to guide Ben (and maybe Anakin) I do not know. It depends on George Lucas and how he wants the story.