The forum user should have the option to format his message as Markdown because

* it's a popular markup exemplified by its use on the popular discussion sites Stack Exchange and GitHub (and with a limited syntax in f.e. Google Hangout and Talk)
* its syntax is significantly simpler than the current HTML markup option f.e. for common markup such as quote, link, list, italic, bold, code
* it's relatively non-costly to implement because parsers exist in most languages