I would have to find my 5D book, but the issue there is that holding the shutter button down halfway also locks in the metering. So when you re-compose you have both the focus and metering locked. With the focus on the back button, you can separate the two, lock them to what you want, and then still re-compose the shot. My background was shooting racecars, so holding the shutter halfway and tracking wasn't exactly something I could do. I could hold the back button down, and then just fire the shutter when I was ready. Lots of panning, so trying to be delicate with the shutter button wasn't really a good idea for me when things got moving.