I'm one of the founders of Perch (http://grabaperch.com) and we have almost 6 years of experience with Perch - and many more of creating custom CMS solutions before launching Perch. We've always tried to get people to write Textile or Markdown.
When we were developing custom solutions we actually had pretty good success with getting our clients to use Textile, once we'd explained the benefits. With Perch we ship our default templates (which are really just samples) using the MarkItUp editor and set to use Markdown, and not allow HTML. However it's all configurable and we offer editor plugins so people can switch to using Redactor, CKEditor, TinyMCE or create their own editor plugin.
In discussing this stuff with our customers who are developing sites for clients, it's not something that inserts HTML that they want. They just don't want the end client to see the Markdown as they don't think they will cope. I think there is often an assumption that they won't cope, but for whatever reason they end up installing CKEditor or whatever and it allows in a load of junk markup. This makes me sad.
So, an editor that created Markdown yet allowed for some sort of preview of how things will look I can see the benefit of, if just to encourage confidence in not using things that allow inserted HTML.