Unfortunately, org-mode is only a portable format if you stick to bare-bones org-mode; if you start using any of the "power-features" unique to org-mode you're stuck. And if you restrict yourself to markdown-compatibilty, why not just write in markdown?
- Extensibility. e.g.: I got tired of manually typing description of links when I insert a link to a GitHub issue, I just wrote a tiny snippet that automatically retrieves the title of that issue.
- Literate programming. For example: I can have a python snippet that when I evaluate, results would be piped it into another snippet in SQL that retrieves data from postgress, and then I can send a http request to a remote API based on what's in DB, and then use that to build a chart. Can you do that in Markdown?
- LaTeX support. I can write some complex math formulas.
- Exporting. I can turn my Org-mode file(s) into a reveal.js presentation, into a book, or a web-blog.
- etc.