All the "plain text" formatting languages tend to break down. They work until they don't and then somebody invents yet another text-based pseudo-markup language. Asciidoc is best of the worst but fundamentally it's a bad idea. If you're willing to make the investment in tools and training there's a lot of value in enabling people to write real HTML (and MathML) or, better, Docbook + domain-relevant XML that produces real UI-agnostic structured information.
The problem here is that the tools are terrible or expensive or bloated. There's no reason why this should be the case and it's likely a real market opportunity.
The spec for Markdown's syntax includes HTML in its entirety, though...
Web development is complex enough (server side, js, css) without adding an abstraction layer to your markup too.
The problem with HTML is it’s verbose. So folks like to save some keystrokes (and mistakes).