This actually works with GNU Screen. Because of this, and because of the lack of workspace reordering in tmux, I seem to be actually about the only person who likes Screen more than tmux.
What do you mean? Commands like tmux swap-window and tmux move window allow you do move the order of the windows / workspaces.
http://superuser.com/questions/343572/how-do-i-reorder-tmux-...
It kind of works in the end, but it's arguably a bit more awkward to use than in screen.
I agree that it does not work 'out of the box', but once setup, it works quite nice :)
Vertical splits were patched in and added a long time ago, screen is usually installed by default, so I know it is there on systems where I'm not allowed to install packages. I can share screen sessions with co-workers, split windows etc.
IIRC, that patch floated around for a while.
The other thing that tmux brings to the table is better scripting support than screen + things like tmuxinator (for me at least).