Years ago I owned a beautiful white german shepherd. In a way the story of the white german shepherd is similar to the parti yorkie. The white german shepherd is a true german shepherd but the parent club decided not to allow them in the AKC conformation show ring so eventually a separate national club for white german shepherds was formed.
History of the White German Shepherd Dog
While, I know that there are many serious caring breeders of parti yorkies, I really don't think the parti color will ever be accepted in the show ring. I also think the bad parti breeders greatly outnumber the responsible breeders but I think it is like that with any breed.
I have never produced a parti Yorkie in my 13 years of breeding and some of my dogs have some of the dogs mentioned earlier in their pedigree.