I got in the habit of using GNU screen and so have never used tmux. But if you have a long-running text UI program (e.g. irssi) you want to be able to connect to on a whim and pass around to multiple client sessions, they're handy for that.
Yeah, I've also been using screen for years without ever feeling the need to switch to tmux. Although remote ssh work is relatively rare for me (eg a couple times per month)