Most people use tmux locally, because their terminal emulator doesn't do what they need it to do, or the window manager doesn't do it. Using tmux locally merely supplements the window manager. I stopped using tmux locally after switching to i3 (and I believe any other tiling / proper wm would do). Now my only use case for tmux is on the server where I'm running weechat. When sysadmining, it's eaiser for me launch few terminals in my window manager and ssh to the same server, then running tmux remotely. It's simply a question of ergonomics, how many sets of keyboard shortcuts do you need to remember to handle a multiplex... eh, window manager running inside a window manager?