mandoc can output markdown, and you can read Ingo’s thoughts on the matter.

The reason for providing this output mode is not that i consider markdown a good, or even a half-decent, markup language. Quite to the contrary, I hereby offcially declare it the shittiest markup language i have seen so far. Basically, it hasn’t any strong point whatsoever, but the downsides are numerous, scary, and cover practically every relevant aspect: