I always advocate for XML. While Markdown is like "a car in any color as long as it's black", XML is a) exactly what you need and b) as precise as you need it, and also c) not more precise than necessary.

We all need headers and paragraphs, right. But I might want file listings. Or command-line snippets with my own color coding and marks. Or C declarations and precise references to them in text. Or todos. Or bibliographic information. Or I want to express the difference between emphasis and italics. Or I don't. And so on. With XML I grow my own notation that fits like a glove. There's no boilerplate, everything is there for a purpose.

Besides, it is not that incompatible with plain text. Here's plain text:

    abc def ghi

Here's well-formed XML:

    <text>
    abc def ghi
    </text>

That's all, save for two extra newlines. Plain text is the best thing when you write to think. But once you're done with thinking you might want to add a date mark, a tag, or another more structured content. With XML you can have both.