The most interesting part of that interaction was Darth Bane saying Yoda had to kill him; similar to certain allege hauntings on Earth, Sith spirits might have to be "invited" in order to manipulate reality. If makes you wonder what would have happened if a someone weaker, a Padawan or Knight, was in Yoda's position. Could Bane possess a body? Potential plot device..... If you recall the KoToR series, the ancient Sith were bound to their tombs and certain objects. Powerful Sith Lords tend to be trapped and unable to achieve death.