i use markdown because it's inherently limited to styling that is easy to represent. it's a good way to communicate the limitations of text-based content submission form.

if you tell somebody they can use HTML, they get frustrated when you tell them that tags other than anchor, bold, italic, list, heading, and paragraph aren't supported. but if you tell somebody they can use markdown, then they implicitly understand that the content they're submitting won't be rendered as green text on a purple background, and don't try to accomplish that.