Doing the same (though new-session rather than attach) - tmux is just part of my everyday workflow. I use multiple tabs/panes/buffers/windows /splits/whathaveyou constantly where I guess you may be relying on your terminal emulator or window manager for the same features?

To me this reads like "As a user of X11, why would you want your session to start a desktop environment on login?"

Because the first and only thing I'd do unless something is broken is run [startx/tmux] anyway.