The add-in argument I think is missing some key factors: add-ins done for fun are not really industrial strength code, or UI design, and are not available in different languages. They haven't been security reviewed. The developer who built it might move on to something else since they are just doing it as a hobby anyway.
Remember that for people who can't code, an API is not providing them value directly. If you tell a customer that they bought a product, but they need to write an add-in to get the functionality they want, or track down on the net something someone else wrote in their spare time, that isn't too customer-friendly. For the great majority of people the feature does not really exist and never will. The Office family of products go to a large audience and not just the sophisticated users you find here who would write or search out and download code. The ordinary person wants things packaged up nicely, debugged, in their language and so on. Of course, you could argue that only the sophisticated users would blog, so no problem. But then there are people who say that blogs are taking off and becoming mainstream - there are millions of them already after all...
FWIW there is a rich API for Word2003, and rich XML support, and even some blogging add-ins, but most people aren’t aware of them. Even with hundreds of custom-use add-ins available for different uses the total number of users do not add up to a great majority of users taking advantage of them. It is still a small minority really even taken collectively, given the huge numbers of units involved with Word.
I'm curious about the lack of interest in blogging support from the people who commented. Is this because there is a belief it wouldn't age well, given that blogs are evolving quickly? Or is it because there's a sense that there are already too many blogging tools available? Or is it that the app is not suited for this? Where's Scoble when you need him?
Today you can use OneNote for blogging if you have a blog site that supports receiving blog entries via email. Greg Hughes is doing that here.
OneNote has an edge over other tools for blogging in that you can post multimedia entries, including handwriting, ink-annotated photos, audio, etc. It is also a natural place to manage a lot of different entries you are working on, especially if like me you tend to write on different topics and take some time on each entry and don't simply post whatever floats across your brain at the moment (not that there's anything wrong with that :-))
I'm surprised that the people commenting were not offering any designs - even of a potential add-in. To me it is too simplistic to say that selecting some text and posting it is what is meant by blogging support in OneNote. Offline use is interesting I think. Also being able to view all my past posts while offline is useful. What about blogging internal to a corporation, where their might be a little more interactive or collaborative use of blogs?
OneNote is used by many people as a research tool even more than as a note-taking tool. People grab stuff off the web and paste it into OneNote to gather info on a topic. Handling RSS feeds seems a natural to me to extend that (as Brian noted in the feedback). It is a nice complement to manual information gathering.
So, any thoughts on what the design of blogging or RSS support would be? Even if you would do it via an add-in, how would it work? Blow me away, please...