I think that “Chat” and “Message” are quite redundant. Sure, they are implemented somewhat differently. But it comes down to the same thing. I suppose it’s up to the forum admin to decide to allow both or one of the 2. I personally would only allow Chat, if that plugin was enabled on my forum. Because Message also has typing indicators and is in a thread. So I personally see no difference. I think it’s confusing for Discourse veterans and newbies alike to see 2 ways to sent a DM to someone. And also, when you have to check your chats with someone, you now have to check in 2 sections?
So I’m all for choice. If an admin wants to disable Message, or Chat for one-on-one communication, then that would be more than welcome! 
Ah, this can already be done with enable_personal_messages in the admin settings 