One of the powerful features of wikipedia vs. topic specific wikis is the ability to spend literally hours bouncing around seemingly disconnected subjects, learning something new at each new node. You can start at Naruto and end up at African tribal weapons in a sequence of incredibly smooth segues that can consume literally hours of time. Almost everybody who does this describes the experience as "pleasant" or "amazing" and all come away having learned something.

A walled off wiki simply doesn't allow this type of holistic discovery. And in many cases, the boundaries between subjects can seem rather arbitrary and fuzzy. Where does Naruto end and Feudal Japan start?