That's what I figured you used. What's wrong with shutter button focus?
Issues with focus/recompose (esp with cameras like the 5d2.)
Being able to adjust shutter speed while focusing is really beneficial to me.
Back button seems to be better for tracking a subject.
You won't accidently fire off a shot or two when trying to focus.
If an object moves into your frame, you can let off the focus and still get it to stay in focus, but with shutter button, you would still have to press the shutter.