I think the key is having the option to use the mouse, in addition to normal Vim keyboard shortcuts. If I'm switching back and forth between Vim and a browser, for instance, I have my right hand on the mouse already, and I can scroll with the wheel, select a tmux pane, copy and paste things, etc. very easily. If I didn't have that option, I'd have to switch back to the keyboard. If I have both hands on the keyboard already, I'm probably not going to use the mouse.