Though I love Drafts A LOT, and markdown is my favorite writing format, I don’t think its necessary for Draft to support markdown preview functionality. Actually I’m quite surprised when I found the Show Preview item in menu.
In my mind Draft is a plain text writing tool, it should just do this one thing and do it well. While text is simple and lovely, markdown, on the contrary, is chaos and arbitrary, there are so many derivatives and extensions, different libraries/softwares have different implementations, which always result in slight difference in the rendering result. Adding markdown preview support too much/deep means Draft will involve all kinds of markdown libraries, the package will bloat, and frankly speaking, it’s hard to do it well, this has been proven by lots of markdown editors.