Steve Rubel on why you should use Markdown:
Video and imagery are critical. But, still, nothing on the web flies the way text does. Words power the web. So it’s critical that every communicator know how to succinctly organize, convey and present ideas for the web and mobile so that they build up readers like nutrients do, rather than weigh them down like junk food.
Agreed and Markdown is a great way to write for the web. Especially when coupled with TextMate.