This was getting into the territory of legends about Kingdom Hearts and the Keyblade War...all very heavy stuff no matter where and when it was said. He never discussed it too much with his own students, let alone with others from different worlds.

"In this manner, it's difficult to say what ought to be and not..." He said, trying to choose his words carefully. "But now, as it has been for awhile, I believe their purpose in remaining separate is to prevent a quick spread of darkness."

If all the worlds were interconnected, a force of great darkness like the Heartless -or worse- would overcome everything instead of just select worlds.