What I was able to parse from the documentation, this seems to be "one way" API, meaning that one can only post new notes to OneNote but there's no way to read notes via the API. I was expecting a little more from Microsoft in 2014.
Just thinking about how I use onenote. Getting stuff into Onenote via an API is much more useful than getting stuff out of it via API.
writing a draft in Onenote and sending it to your blogging CMS, syncing a todo list in Onenote with a 3rd party project management service or task manager. Parsing a page in Onenote and retrieving meta data via another API to add to the page. Most of all, allowing the data to flow freely between the tools you use. There are tons of interesting things that become possible if OneNote fills out a more complete API.