I disagree. I know I'm on the fringe but I make my living as a writer and 100% of my work is written in Markdown (MultiMarkdown, actually) that I then convert to HTML to paste into WordPress. Is it perfect for everyone, no, but for the subset of users who want to roll their own server setup and still have an easy-to-use publishing platform, it sure does fill a lot of boxes.

Moreover, I can teach people Markdown 10x faster than I can teach them CMS-specific HTML rules. Moreover, WordPress's TinyMCE stuff can break quiet easily (we have to disable it for most of our writers, I'm still allowed to have it because I never use it), so having a readably syntax AND having an instant-preview seems pretty perfect.