On the road on a modern bike I think it probably is nonsense, but each to their own. I move in the seat where things slip or if the bike has loads of power, but more off road style, back and forth, weight the outside peg etc.
The Bonnevilles stand scrapes first then the footpegs. The MZ puts the stand down as well, both when your knee is about a foot off the deck, so no chance of any antics with knee sliders there. Both the above and every bike I owned over 100cc would break every speed limit in the country, had rubber that (in the dry) would allow you to almost lay the thing down and non were capable of stopping in the visible distance on most roads if you took them to the point where you might need to lean off to corner at the best possible speed. There is also the question to me about the ability of a road rider to change direction, many roads having things like kerbs, lamp posts and changing surfaces, not gravel traps. If you hang off to the left and someone has left a skip or a dead sheep can you move right? You can if you you are central on the bike and know the limit. Obviously not a concern on a track where you set up for each corner, but to me a real issue on many roads where you have any chance of riding like this.
I'm therefore sort of glad that all those blokes wearing knee slides are making the scuff marks with a black and decker (or on tracks), it proves that while they might be daft enough to want to pretend they are Rossi on the road, they don't all want to end up in body bags and most might actually be riding to the road, not thinking it's the TT.
As a track skill I think the knee down approach is maybe easier to learn? You just take the same corner that the instructor knows 5 mph faster each time until your scraper touches? The alternative way involves finding the maximum speed/angle/slip and probably involves falling off? I know that the track day I did taught me a lot about adhesion (but not half as much as off road days), so perhaps we should all learn how to do it. It would sure help a few of the rice rocket muppets who'll pass you at 100 mph on a B road and then slow to walking pace at the first corner. After that, why do you need to?
My big dissapointment BTW was BMW twins, the stands scrape first there as well. Still, we had a good laugh with the power rangers at work mounting sliders a mate had used on a track on the engine bars! A few even believed it after I had the sidecar mounted and had done a bit of chair flying in the car park!!
Andy