i dont see much of an advantage over just holding the button half way down
The disadvantage of using the half-press shutter button is that it starts the auto-focus and
locks the auto-exposure.
If you use servo-AF and track a plane for a long time it's likely that the exposure that was set when you first half-pressed the shutter button is wrong for the conditions when you take the shot. Putting the AF control on a separate button allows you to have focus tracking but set the exposure at the moment you take the shot.
As an aside, the default behavior of the *
button is to fix the exposure, and then the half-shutter sets the focus.