I think the real use case for me with OO and markdown is to get my outlines into Github. Personally, if I'm going to write in markdown, I'd use either a text editor or, for bigger markdown writing projects, something like Ulysses. I love the cleanliness of UI with OmniOutliner as is, but have always had to walk away from using it as a regular tool in my workflow due to limited interoperability with my other tools.
Happily (but unfortunately in this case) nobody on my dev team are huge Word users - we're all text editor die hards in some form or another - rendering Word export a feature that isn't very useful for us. What we really need is a nice clean way to export to our Style Guides (which sit on Github) - including wrapping images into an images folder and building markdown-friendly references to them. Honestly that would crush when it comes to dealing with the time it takes banging out that very specific type of work.
I dunno, just thought I'd share my biggest use-case for markdown exporting.