What DarkPenguin said, plus: flipping camera around by the L-bracket is much faster than flipping the ballhead over to it side and recomposing (the camera stays pointing in exactly the same direction with the bracket).
This is the most important
aspect of it for me.
If the camera is level in landscape mode and I flip to portrait mode, it's still level. There is no risk of a one-degree error just because there was something preventing the ball head for flipping completely over, and so on.
I was so irritable when switching compositions earlier, but after I got the L bracket for my 20D, it's been so incredibly easy every time.
In addition, I find that it provides a nice extra grip; I don't use the BG-E2.
The downside is that if I wanted to use the BG-E2, I'd need a second L bracket, and there would be lots of hassle switching between using and not using the BG-E2.
The same problem goes for the 5D and the BG-E4, of course.