Your conflating the medium with the source of the message in this analogy.

Writing and oration are two different media, the question of which is preferable in which context is completely unrelated to LLM authorship.

If "the purpose of writing is to convey ideas" (which I largely agree with), which ideas are being added between whatever you prompt the model to convey and what the model conveys? Are you proposing that an LLM can extract some meaning from your initial prompt that a human being couldn't?