I do it, because I like braking hard. By 'it', I mean I accelerate hard, brake hard and repeat x5 / 10 times. Then I touch the disc, think "ow, I won't do that next time" and ride. It's mostly so that I know I've set the brakes up properly.
I asked someone about it who was involved in F1 and the manufacture of brake pads (among other components) for fast cars. He thought the idea of us lot bedding in pads was hilarious; the pressure and temps that the pads undergo during their manufacture are many orders above what we can create. He said that we might burn off a little residue from the factory but that that would happen anyway; there certainly wasn't any long term benefit.
However, what would an automotive engineer at the top of his game know compared to the average STW'er
The other two thing's he said were 1. buy a red bike for 10% extra speed (that's why Schumacher won for so long) 2. don't wash your shorts before a race and always put your right sock on first or you'll never win!