Why is a registered AF point useful, will the camera change from whatever focusing mode it is in to the registered one with the touch of a button?
The AF mode doesn't change, but the beauty of the registered AF point (and I use this option all
the time) is that simply by turning the camera from landscape to portrait orientation, the AF point will instantly jump to the preselected position you've already decided is best for portrait mode; and when you rotate the camera back to landscape, the AF point goes right back to where it had been when you were shooting in that orientation.
Shooting birds for example, this ability saves vital seconds which can make the difference between getting the shot and not.
Mind you, it'd be more useful again if the 7D's grip had a built-in joystick a la
5D Mk III so that you could fine-tune the AF point selection in portrait; but even without that, the automatic switching to pre-chosen AF points is a Godsend.