This question has been discussed awhile ago. I uses it on my Nikon system all the time, except when I need to pass the camera to a stranger whom has no idea how it operates. You set it to perform AF and your shutter release to only release without AF. In this way, this will allow you to release the shutter without refocus all the time.
The following as some advantages through my personal usage that I feel is useful.
1) Allows recompose even in continuous shooting mode
2) Useful in fast motion scenes such as sports, racing and fast moving subjects where you can you release the shutter without refocusing. Eg: In the event for soccer game when you can see a player is going to cut your line of sight when you are continuously focusing on another player further away, you can quickly release the AF button and the AF stops, the focus is likely still on the player when the intercepting player moves away. You can then capture immediately, or press the AF button again with less chances of focus hunting.
There are some quirks of course, because the shutter release no longer requires in focus to release, there are higher changes of out-of-focus should your subject is not in focus when you release the shutter. You will need to depend on your hunch, or visually on the in-focus indicator found in the viewfinder to indicate that you are in focus before release the shutter. This is not so much a problem for a slow photography event such as landscape or event, but for fast motion event, there is higher tendency for OOF.
All my old 5D and 1Ds are set to back button for focus... mainly for easy lens manual override the and to overcome focus hunt. other then that I can depress the shutter anytime without the lens trying to focus thus missing the shot... but you have to practice and get use to it...