Ong, quoting Havelock, maintains that Plato's disdain of poets is from his awareness of the change of civilization from orality to literacy.
But, by Plato’s day (427?-347BC) a change had set in: the Greeks had at long last effectively interiorized writing - something which took several centuries after the development of the Greek alphabet around 720-700 BC (Havelock 1963, p. 49, citing Rhys Carpenter). The new way to store knowledge was not in mnemonic formulas but in the written text. This freed the mind for more original, more abstract thought. Havelock shows that Plato excluded poets from his ideal republic essentially (if not quite consciously) because he found himself in a new chirographically styled poetic world in which the formula or cliche, beloved of all traditional poets, was outmoded and counterproductive.
However, it is not that Plato changed himself from a man of orality to a man of literacy once and for all. As is often the case in times of great changes, Plato cherished and loathed the new culture of writing at the same time.
The conflict wracked Plato’s own unconscious. For Plato expresses serious reservations in the Phaedrus and his Seventh Letter about writing, as a mechanical, inhuman way of processing knowledge, unresponsive to questions and destructive of memory, although, as we now know, the philosophical thinking Plato fought for depended entirely on writing. No wonder the implications here resisted surfacing for so long. The importance of ancient Greek civilization to all the world was beginning to show in an entirely new light: it marked the point in human history when deeply interiorized alphabetic literacy first clashed head-on with orality. And, despite Plato’s uneasiness, at the time neither Plato nor anyone else was or could be explicitly aware that this was what was going on. (p. 24)
Another interesting part in Chapter for me was where Ong mentions Julian Jaynes. Ong picks up Julian Jaynes as one of the few scholars who clearly understands the difference between orality and literacy (Remember that most modern linguists regards writing as a mere transcription of speech).
However, if attention to sophisticated orality-literacy contrasts is growing in some circles, it is still relatively rare in many fields where it could be helpful. For example, the early and late stages of consciousness which Julian Jaynes (1977) describes and relates to neurophysiological changes in the bicameral mind would also appear to lend themselves largely to much simpler and more verifiable description in terms of a shift from orality to literacy. Jaynes discerns a primitive stage of consciousness in which the brain was strongly ‘bicameral’, with the right hemisphere producing uncontrollable ‘voices’ attributed to the gods which the left hemisphere processed into speech. The ‘voices’ began to lose their effectiveness between 2000 and 1000 BC. This period, it will be noted, is neatly bisected by the invention of the alphabet around 1500 BC, and Jaynes indeed believes that writing helped bring about the breakdown of the original bicamerality. (p. 29)
However, as is already clear in the above quotation, Ong seems to believe that the bicameral hypothesis can be reduced to a simple argument concerning orality and literacy.
The Iliad provides him with examples of bicamerality in its unselfconscious characters. Jaynes dates the Odyssey a hundred years later than the Iliad and believes that wily Odysseus marks a breakthrough into the modern self-conscious mind, no longer under the rule of the ‘voices’. Whatever one makes of Jaynes’s theories, one cannot but be struck by the resemblance between the characteristics of the early or ‘bicameral’ psyche as Jaynes describes it - lack of introspectivity, of analytic prowess, of concern with the will as such, of a sense of difference between past and future - and the characteristics of the psyche in oral cultures not only in the past but even today. The effects of oral states of consciousness are bizarre to the literate mind, and they can invite elaborate explanations which may turn out to be needless. Bicamerality may mean simply orality. The question of orality and bicamerality perhaps needs further investigation. (p. 30)
Anyone who's interested in Julian Jaynes' theory of consciousness, cannot afford to ignore the media studies.