This is a decentralized bit of data that any interface can use.
Just like the profile description is a decentralized bit of data.
I personally would hate profile descriptions if people could go all crazy with the size stuff. It's just meant to be a quick bit of data not a big presentation.
While there's nothing against markdown per-se let's look at some of the following:
We should realize that this is mostly designed to be A "short description"
If there was a need for a long description or a full about page then i could see the need for mark-down
And looking where it will end up in our interface and others means it has a small space that it occupies or it may end up being inside a popup.
With our UI if we allowed markdown, we'd just have to develop something that got rid of markdown because we can't fit it where it's at.
You could make the case to future proof if we feel like it could easily move toward a longer description... (however we do link posts)
But in the future if everyone does a description and they become very important then I suppose we should give it more high priority. If there's a demand for it to move from short description to a medium one... then yes markdown could maybe make more sense.
Remember long descriptions are meant for the URL link post.
In the end it makes sense to wait for more interfaces to start implementing the description and see what the witnesses want to do.. maybe they'd like to have a medium description. However i feel like users still want to have a short description.