Earlier quoted context omitted.
I used to have 2 pcs, connected over synergy, for the same reason. The best part is that your cmd-tab / application switcher is local to that screen. Still looking for a similar solution on 1 laptop with 2 screens.
I've longed for a window manager that can assign different virtual desktops to different screens. Of course, there are compositing, scaling and resolution issues to overcome, but it'd be really neat to have a palette of virtual desktops that could be called up on whichever monitor was most convenient. ...and it'd make screen mirroring during presentations a breeze!
I have my left monitor as a communications hub. It has only one virtual screen. I also keep my browser there.
I have a 3x3 grid on the right hand monitor.