It really depends on how you look at things...
Sure, you can ride on the street with never shifting your butt on the saddle even a fraction of an inch. You can do ramps, you can negotiate corners and all the other general aspects of riding.
You claim you should never do anything except practice driving on the street when you're on the street. Look at the
Lee Parks Advanced Riding School...They're training you for street riding, but what's this? They're teaching people how to get off their bike and use their knees as a lean angle gauge? If shifting your weight and "hanging off" your bike has no place on the street why on earth would they even bother teaching it? Simple reason - to make you safer. If you're taking a turn at lets say 35 degree lean angle with butt firmly planted on seat, you'll be at 25 degrees (rough estimation, no factual science). Most sport bike tires lose traction beyond 45 degree angle. This leaves you with a 10 degree safety margin with butt on seat, and 20 degree safety margin with shifted weight. Personally, I'd rather have the extra 10 degrees for emergency maneuvers if necessary.
油井緋色 is completely correct - bikes are more stable when upright, so why not increase stability by shifting your weight?
If you have a higher skill set than most average joe blowhards from riding track or taking advanced riding schools, why not make use of those skills? I'm not saying race into corners and behave like a hooligan, but why not use those skills to make your riding experience even a tiny bit safer?
Most people I know who have ridden track for a couple seasons shift their butts around on their seats when taking turns on the street. They don't "hang off their bikes to a ridiculous degree".
In general, if you've got more than 1 butt cheek off the saddle you've moved too far, even on the track.
Looking forward to reading your rebuttal.