It might be quite similar window/tab managing functionality, but for me it's the same thing that made me choose tmux over screen: it comes with a nice status bar as default and hotkeys are somehow easier to memorize.
My dtach+nvim uses nvim as terminal multiplexer & "status bar", which is pretty ok! Dtach only serves as a very very dumb pass through (where-as tmux really is a persistent virtual terminal that clients can read-out from).
