The general problem, of course, with WYSIWIG is distribution: what YOU see might not be what someone you send the document to sees. Do the people you're sharing the document with have the same application, the same version of that application, the same fonts installed? You can export to a presentation format like PDF (or HTML), but then they can't edit it as you did, and it's also likely that the translation will change "what you see".
Markdown is a least common denominator for sure, but that limitation also gives it a useful universality, which is what proponents value in this case.