[mhink]

The one major advantage is preserving sessions and re-attaching to them later, e.g. when you're ssh'ed into a remote machine or VM and you don't want to lose your setup.

Ratpoison could possibly be better (I keep meaning to give it a shot), although one thing I really enjoy about using tmux is being able to quickly open, shuffle, reorganize, and close panes with only the overhead of starting a new shell.