Authors, Illustrators, and Author-Illustrators

In an interesting interview with author-illustrator Tomi Ungerer today, he claimed that all the classic children's book writers are author-illustrators, not "merely" authors or "merely" illustrators.  Specifically, he said, "Look, it’s a fact that the children’s books that withstand the grinding of time all come from authors who did both. Because the author has a vision, and there’s an osmosis between the oral and the visual, which come together and mix." Do you agree?  Can you think of counter-examples?