This manifesto discusses how speaking and writing have become separate disciplines over the last 100 years and how members of both disciplines tried in the 20th century to reintegrate them because they have similar objectives. According to Clyde Dow, these are "clear thinking, judgment, evaluation, organization, methods of development, vocabulary, good informal usage, and knowledge of how language influences others and ourselves" (qtd. in Mt. Oread Manifesto). I can definitely see these objectives manifesting in both disciplines.
The main difference is that writing instruction tries to achieve them through the written word, and speech focuses on verbal delivery. It is as if the same thought processes can go into both activities, speaking and writing, but the method of delivery is different, and each method carries its own intricacies. For instance, the idea of using a clear vocal speaking style is part of the speech discipline; in writing there would be more emphasis on a clear writing style. There is somewhat of a difference between speaking clearly and writing clearly. However, both can involve clarity of thought processes. Still, it does make sense to integrate speech and writing to some extent! They are both crucial aspects of human communication that pedagogically require emphasis on thought.