While Plato may have in fact harbored such beliefs, I don't see the sources for where he actually says what you say here. Is this your interpretation of his dialogs? You see, Plato was very concerned about the written word and because of the dilemma of Socrates, rarely came out with direct statements. Instead he would couch his ideas in the dialog between characters in his writings. This served two purposes: one to put some distance between himself and accusation because of his writings and two, to keep the written word alive, because often, the spoken word is the "dead word". This statement was actually given to me by Robert Schmidt, a classics scholar and translator of ancient Greek (and several other languages).