You mean like vim, emacs, nano, kate, notepad, notepad++, wordpad, or any other plain text editor?
Really - I don't get this impulse at all. From my perspective, the entire point of markdown (and RST I assume, though I haven't used it) is that you can easily read and write it using any plain text editor.
If you are going to need a specialized editor to write it, you might as well use html, docbook, docx, odt, troff, rtf, or any of the zillion other formats.
Back to the main point, for non-developers all of those can be quite confusing especially when you're used to Word/Google docs/etc. A text-only editor where you don't get live feedback is quite an adjustment and there is a cognitive switch to the text you're writing and imagining how that text will be presented.
It's been a while since I've written rST but there were good plugins for both emacs and Sublime to boot (for e.g. filling the header under/over line or toggling between different header styles)
