I am n00b when it comes to enneagram, but I just wanted to make a comment on categories in general.
ALL categories/types/whatever are essentially arbritrary. This is true in any field. There are constraints on how you can make a categorical system where things are mutually exclusice. But it seems like MBTI and enneagram are both fuzzy on the boundaries they use.
Categories are conceptions of the human mind. They ALWAYS are. It is a question of whether or not the categorization is usable or not as way of describing reality.