Excellent! It is the most beautiful picture isn't it?
I think the PK method is much more demanding (on the rider) and more subtle than the conventional route, so I guess many people don't have the patience to pursue it, even though the argument from the horse's point of view (in terms of...
There's a picture of a perfect piaffe (the grey horse) at the bottom of this article by Philippe Karl. It's also an interesting read if you're not too put off by the dodgy translation and the baffling maths formulas...