Now that I know what button on my mouse this is, I have to say it's SUPER handy. my thumb already rests on that button when I hold it so I never have to take my fingers off WASD when I am in a tense situation like being attacked. A good move is to switch to your preferred heal, and then to your primary. When you get into trouble, your heals are a millimeter away from a thumb press, you never have to leave movement keys to switch to a different hotkey.
While it was a bit awkward at first, I find myself using it constantly even when I am out harvesting, switch to your preferred hydration/coffee and then back to your ax/pick and boom quick switch, it's easier and less fatiguing on your hands overall.