Lets not be too precious about literature meaning marks on paper. Much of what we call literature started out oral. Homer and Sappho, as the Nobel committee pointed out, would have sung or intoned their work and Homer- the blind minstrel- was almost certainly illiterate. Even today a lot of literature is designed to be performed rather than read. We call the great playwrights literature, we call the Book of Common Prayer literature, we call the Gettysburg Address literature- so why not Dylan?