Justification: Github Pages has turned markdown into the lingua franca of rapid content creation. A large subset of developer now know & regularly use markdown for quick writeups of internal docs, wiki-style pages, etc. Ability to author pages in some flavor of markdown would be a big value-added for us on Zoho Projects.
Other comments: Non-breaking UI for this could be implemented by extending the "Toggle HTML Source" button in the top-right to be a drop-down.