That's not a real rule, and may even be a horrible rule depending on a person's controls. For example, if someone uses mouseover targeting or click targeting, mouse movement is a bad idea. Remember Fitts's Law. If you're moving around with the mouse, and then you have to mouseover target someone, you're not going to have an easy time acquiring all your targets and also moving because your mouse will be back and forth traveling greater distances than it would if you were just keyboard moving. It's also possible to do combat in this game without a mouse at all. It's incredibly unusual to need the level of precision camera control that a mouse gives you, and the keyboard works fine. To date I can name one fight where I've needed the mouse to aim my character precisely (E12S's Shiva phase, for a precision ice floor slide). Every other fight keyboard movement and camera controls are good enough.