I started using it mainly due to another reason apart from the one commented here. If you use focus&recompose with evaluative/matrix metering, you should separate buttons, otherwise metering will be wrong. When focusing and recompose, half press shutter will also lock AE under this metering mode, but the recomposed scene will likely have a different metering. With BBF you get rid of this problem, and also gain the benefits described above.
However, I have to say that I don't feel as comfortable as with the half shutter, I sarted using it few weeks ago though. My problem is that pressing the back button make you move your thumb, which is very important to firmly hold the camera. So I feel that I cannot grab the camera as stable as before and thus precision in focus may be affected. Also, it is not as quicker as half press. Time will say what method I use the most.
Meanwhile, I would like to do a suggestion to Canon. Why one would want to have duplicated functions in shutter and AF-ON by default? It would be smarter to have a timer when pressing AF-ON that deactivates AF Lockr from shutter, lets say for 10 seconds. That way I can use half shutter if I want to go simple or fast, or AF-ON with focus and recompose, both at the same time without having to go to the menu to swich button functionality. That would be a very easy firmware tweak.