What an excellent question. I've been told in the past when I've "hopped" that it was incorrect or rather incorrect for my level (I'm currently a nidan in Yoshinkan). We are constantly told to remain grounded to provide the power necessary to achieve proper technique. With regards to Kancho and his almost balletic spins, I believe it may have to do less with wanting to "hop" as having to, as many of his uke were taller than him, although my teacher, Kimeda sensei, is also shorter than I am and I've never seen him having to rely on "hopping".
Maybe when we achieve a certain level it's allowed, since I've been told that the movements may become tighter as we progress and the pivots shorter which may give the illusion of hopping in some cases.