I think that sounds like how they worked pre-sidebar/pre-enabled_groups setting. With PM’s disabled for non-staff users a user would get a notification, but no access to the message tabs in their menu or user page (though they could still access them directly if they knew the address).
Though I think it works slightly differently now for the similar direct_message_enabled_groups, so this may be something to marry up and make more consistent.
I think that’s reasonable grounds to revisit this, but the messages UI comes with more complexity right now (with it’s inbox, sent, etc). And you can close a direct message to remove it from your sidebar, whereas that’s not something you can do with messages.
Agree this is a bit of an odd duck, but we may let it sit for a bit before we dig into making any changes here.
Some of my users find it really confusing; after reading the private message they are unable to locate their PMs; despite their inability to send new PMs. Hopefully this can be changed to something more clearer for our users sometime =)