Who knows what they were thinking when they wrote that tutorial... maybe they were a portrait expert... because their job was taking license pics at the DMV, and all they needed was manual focus... lol.
I also use autofocus, I cannot see the advantage of manually focusing it over a fast autofocus lens. Also, you will RARELY be able to get a pic as sharp as fast manually... I do not care how good your eye is or how fast you can twist that focus ring.
Only exception, as mentioned, if light is too low, but thats when I slap on the SB-800, and use it to focus, but manually not set the flash any stronger than 1/128th... enough to trigger the strobes but not add anything to the picture.