Should we add a reason for closing questions that are discussions, or that try to seek discussion?
"primarily opinion-based" may sometimes not work.
Here's a suggested reason, right from the help center:
You should only ask practical, answerable questions based on actual problems that you face. Chatty, open-ended questions diminish the usefulness of our site and push other questions off the front page.
(thanks @5chdn for idea)
Whatever answer gets posted, the asker will probably discuss in comments (as they've done)...
Another example: Is the DAO incident being investigated by law enforcement?
There are questions that may "trap" answerers into giving an opinion or primarily opinion-based answer. This meta discussion is about how to deal with those questions, because even an "agreeable, logically defendable answer" can still attract debates and further discussions...