fast guys have slow hands Richie was a pretty decent racer https://www.youtube.com/watch?v=2qkRiytX5lQ
He made a great point in that interview, too. The clutch is a direct means of cutting the drive, more so than the rear brake or throttle. Also helps for anti-wheelie, when the front starts to come up, just dab the clutch a bit to bring it down.
When you have time, I'd be interested in the reasons. Not to argue, just to get another point of view. My current understanding being that using the clutch to up shift while leaned over will prevent chassis upset/tires loosing grip.