Darkle your example is a good one, title is unneccesary.
but the way i see byobu is its not actually a dm or private messaging system. its fundamentally acts as "private discussions", and its actually act just like that in this forum. thats why you need a title because these "pm"s are listed on the discussion list. im cool with what it offers but i dont believe its what most users would want or expect.
there is an extension for flarum called "whispers" which functions as what i would expect as a dm system. but its not supported right now and has full of bugs.
and im not okay with basic private messaging not included on the core flarum.