I think there's a fine line with what you're talking about. I think you can still play with blazing speed in a musical manner and there are a lot of guys who have made a living off just that. I think if you can shred when it's called for it adds a little something to your skill set that a lot of guys don't have, especially if you can do it clearly, with good feel, and with class. I have to admit though, the guy in the vid is overkill. As for the why, if I (or most other people) had the chance to prove I was the fastest guitarist alive and I could pull it off I'd do it, if for nothing else than the title. Anybody would like to be the best/fastest at what they enjoy doing, that's why we practice like we do, so we can rock out, not just play quarter notes all day long