I'd be happy with a Copy As Markdown function, or something like that. I don't really mind the rich text, since it's simpler to look at than multiple panes, but the end result of something like a draft blog post will end up as markdown for me. Having the markup converted would be nice.
Markdown support means that I already know how to use it. I don't have to figure out what hotkey bolds text, underlines text, turns things into bullet points, etc.
How do I add links to this? Is it even possible? If it was markdown, I already know how to add a link, no extra thought required.
I wish papier would support bbcode. Then I would know exactly how to bold text or add hyperlinks to other webpages on the world wide web (mostly angelfire websites).
I don't mind that this doesn't support markdown, or have super-specific instructions, or w/e. It's a fun bit of software, and I'm going to play around with it for a while :)
I'm just answering the question of "why would you want markdown support?"