Hi CJS,
Separating the Focus System from the Exposure Metering and, more importantly, the shutter action, is brilliant. The half press on the shutter button locks the exposure.
I can now set my focus once and take as many pictures as I want because pressing the shutter button no longer changes my focus. It still meters correctly, but it doesn't refocus. Obviously if I need to refocus because something (wildlife/birds) has moved then I refocus.
Also found this on the net as well:
1) Focus and recompose- With back button focusing, photographers can quickly focus their subject, leaving it on the center AF point which is usually the most accurate, and then recompose the frame and shoot without the camera trying to re-focus with every press of the shutter. From experience, I know how annoying this can be when every shot you have to focus, recompose the shot with the shutter half-pressed, take the shot, then start over again. You could always leave the shutter half-pressed, but who wants to do that for a long series of shots? With AF moved off the shutter, you can focus one time and then take as many shots as you need while recomposed without having to worry about the camera re-focusing.
2) Timing your shots- I typically call this "prefocusing", and use it all the time when shooting sports or other action. Essentially, you can prefocus on a spot where you know action is going to happen, whether it be a batter at the plate, or a runner sliding home, or even a speaker standing at a podium. By prefocusing, you have full control to just shoot away when the decisive moment happens, rather than sitting there with your shutter half-pressed waiting and waiting. With back button AF, you can just set it where you want it, then relax until the moment comes. No worries of waiting on the camera to re-focus when pressing the shutter for that crucial shot.
3) Stopping focus when necessary- This is another that is aimed at sports and wildlife shooters. With back button AF, you can easily stop AF when something might interfere with your subject and cause your camera to try to lock focus on whatever that object may be. Just release your thumb from the button when you think that there may be an interruption (referee, tree, opposing player, etc...) that could come between you and your subject.