I thought one of the most important features of a ball head was ease of positioning. I don't have an L-bracket for testing, but I'm guessing that moving the ball head and recomposing is at least as fast as dismounting, remounting and recomposing with the bracket. And far cheaper. And the camera is lighter for the hand-held shots (I'm assuming most people who use an L-bracket leave it on). It's not like you can leave out the recompose part - I haven't seen a picture through my viewfinder that didn't require a little re-composition from landscape to portrait mode.
Sorry, I still don't get it.
With an L bracket you will keep the centre of your camera in line with the centre of the tripod, where if you flip it on a ball head it moves around the ball giving you a different centre which is then different from the centre of the tripod. Means you can end up with a slightly different angle on foreground objects and can throw off the alignment with the background. To get the same pov you'd then need to move your tripod where with an L bracket you only have to adjust the vertical axis.