We had dozens of community members upvote a feature to add markdown and we added it because it made sense.
Now we really love it and can't go back: http://www.mattermost.org/open-source-slack-alternative-adop...
Headings have a lot of uses for emphasizing text, and there's fun uses too like creating different sized emoticons: http://www.mattermost.org/wp-content/uploads/2015/09/sheep.p...
What made sense? Is there a use case that requires full Markdown support? If so, what is it?
The reasons I use these are:
1. Rich content, organized & clean lists with outbound links for status updates about things I'm working on, linking to Asana/Github
2. Notes from one on ones, or general progress updates that I keep in a private channel and save basically as a stream-of-consciousness/reality log, having them there makes them readily accessible in product and having them in Markdown makes them very readable when I'm browsing through.
In short, I treat Slack rooms almost like I would treat a folder in Dropbox, they just happen to also include some chat between people :P