Didn't Yoda actually say to Luke, "When gone am I, the LAST of the Jedi will you be..." ? If we suddenly are told that Yoda was just wrong, and that there actually always were several Jedi that survived, I think it would completely undermine the importance of Luke passing on his knowledge, and "carrying the flame" of the Jedi into a new era. We were made to understand for years that Luke was in fact the last one left, and it was up to him to restore the Jedi order. If we're then fed some "well, actually..." story about the fact that there were always others, that would ruin it I think. It would cause one to think, "Then it was NOT up to Luke to reinstate the strength and presence of the Jedi perhaps...Why not just one of those OTHER ones???