The left click reorients your facing, which defeats the purpose of camming around to see who's shooting you in the backside if you're still shooting at another target. The button to lock the hotbar doesn't have to be ON the hotbar. It could be something we could checkmark on the expanded hotbar menu that we're dragging skills from. In fact, that'd be an excellent place to put it.
Really? Even with default settings my left click never changed my facing, I had to hold down both mouse buttons to do that. And even if it does for you, you can go into keybindings and change it.