SilverWook, if we were only basing our conclusions on Yoda and Anakin's ghosts, then we could speculate they only appeared on that one occasion to smile at Luke and then never came back. But Obi Wan's ghost has been communicating with Luke for years (ever since the Battle of Yavin).... why would he all of a sudden stop appearing after the Battle of Endor?
If anything, he's only been getting less ghostly with the passage of time. He progresses from being just a voice in ANH, to being a ghost who fades in and out floating motionless in TESB, to actually walking around things and sitting down in ROTJ.
By Episode VII, he'll probably be getting skin grafts and building himself a hut and giving piggy back rides to Han and Leia's kids.