This video is a great demonstration of digital clean-up. If nothing else, the drawing is awesome, feature quality drawing.
However, in the old days (the 90's) a clean up artist would be much more skilled. Drawing on paper forces you to be better. You have one chance to make a good line and no undo button. If you're working on your drawing skills, put the Photoshop away and use traditional media. It'll make you a better digital artist is you motor skills are finer tuned. Plus, you'll feel way more confident in what you produce, knowing that you're not relying on digital's "cheats" to "make you a better artist."
(FWIW, I'm not criticizing the artists in this video. Digital has it's own techniques to learn and finesse to produce quality material. And these guys aren't slouches.)