The rationale I'm seeing by having Category:Theories is that Theories fall under Fanworks. Which gets into a bit of a murky area since there is a Fanfiction page. In the end, we'll need to decide if Fan Theories is something the wiki is hosting under Fanworks. Or if Fan Theories should be encouraged to be posted elsewhere.
The TWI community seems strongly entrenched in supporting Fanworks. If we want to allow for Fan Theories, we should at least have some rules regarding whether a theory deserves its own page, i.e. the theory being comprehensive, showing thought was put into creating it. And longer than just a few sentences I hope!