Well, I am way late to this discussion, but there are two things that I haven't seen mentioned in this thread; maybe I just missed them.
For me, another advantage of using the back buttons for focus rather than the shutter release half-press is that I can flip back and forth instantly from using my registered (center) focus point and all 45 focus points (auto selection). I have set up my 1DMkII so that the X button (the leftmost one) uses the registered center focus point and the * button uses what I have set with the :::: button (the rightmost one), which is set to auto-selection from all 45 points. Since I shoot motorsports, I am almost always in AI Servo mode, tracking a moving race car. Most of the time, center focus point is my preferred mode, but sometimes I want to set the servo focus to track the front of the lead car but let it drift a bit out of the center of the frame while still tracking it as the primary focus point to better balance the frame. I can press the * button while the front of the lead car is in the center of the frame and press the shutter button after the front of that car front is no longer in the center but still within the focusing oval. Does this actually work? Well, it does in theory at least. I think that, along with some frames of the same car(s) focused with the X button, it gives me a wider selection of varied "in-focus points" from a burst of shots from which to select my "keepers" if nothing else. I can always crop to make my focal point not be in the center of the frame, but if I can accomplish the same thing without giving up pixels, that's even better.
Another, perhaps small, advantage of using the back buttons to focus is that it takes the sometimes overactive shutter button on the vertical grip of the 1DMkII totally out of the picture without the need to return the camera to Canon for button calibration or to worry about firing off a bunch of unintended shots. The secondary shutter button on my camera is a bit on the hyper side, but it isn't an issue for me as long as I use the back buttons for focus.
In addition to setting CF18 to setting 2 so that I have to hold down the back buttons to track focus, I also use CF4 at 3 for the same reasons Nill does. The cars I photograph are constantly moving through varied lighting conditions during a single burst of frames.
It does take a while to get used to focussing this way, however. I can completely sympathize with the person who said he fired off unfocused frames while in this mode. I did that more than I would like to admit when I first switched to this method of pre-focussing my pictures. In the end, for me at least, the thumb training learning curve was worth it.