Applying AF to the AF-ON button and leaving the shutter button for only the shutter increases speed and is really aimed at sports photography. I found the following article on Canons website which also references back focusing and being critcal for photographing F1 racing.
I photograph a lot of sports and motor racing and this is what I do (although I have to use the AE button as my camera lacks a dedicated AF-On button ). I find I'm more likely to get the right exposure this way. The only downside is that I'll sometimes pick up a camera, press the shutter button and wonder for a few seconds why its not focusing.
Everyone has their own preferences. However this doesn't make sense with a 1D Mk IV b/c the camera here will meter through the active autofocus point, so you might as well focus, meter, and take the picture with one button vs. 2.