That's not a requirement at all. You can message literally anyone on FB, conditionally - there 'message' button is visible (exception to hackers). You can also message Zuck, even though you may not be friends with him.
You can "message" anyone you want, except that if you aren't friends it will almost always fall in some weird secret hidden bucket that most people never look at, and is hard to find even if you try. If you share a direct connection it might go through, but in my experience not always.
YMMV! In the very recent times, I've sent countless messages to members in various Bronco groups and sellers in the marketplace, I've almost always got the response.
Try messaging someone Facebook doesn't think you have a reason to message. It will go to a different inbox that I bet half of users don't know exists, and even those that do check it once a year.
Isn't that exactly the problem? If I can't be reasonably sure that a message I send will be seen by the recipient, the messaging service is useless to me.
Messaging a random person does.
Point is I want a messenger contact list that is distinct from my FB friends list.