Writing in Markdown is not always easy. If you are providing an interface, an editor of any form, you shouldn't force your users to write in markdown. Especially if you are expecting them to use your application on a mobile device. It's a complete mess on the smaller mobile screens.
Every time I've to write in Markdown on my smartphone, I shudder. Especially if I've to insert a link. Do not make me type all the markups. It is just not easy.
It is a lot simpler to select some text and tap a button, either to make it bold or insert a link. Sure, insert the markup in the background or in the editor, I don't mind that. Just don't make me type the whole damn stars and brackets sequence.
Don't get me wrong. I love Markdown. It is the simplest markup format, a lot better than writing the posts as HTML. However, it's not too intuitive to write on the smaller screens, without a physical keyboard.
So please don't force me use that. Provide me an option of a toolbar.