As background, there has been interest for a long time in having “affinity” groups on MDN, based on areas of interest. Such groups are pointless without a way for members to communicate. But the MDN dev team has been reluctant to implement a messaging system within MDN, because there are so many issues involved, making it a very large effort. Using Discourse with tags might be one way to let affinity groups communicate without building a huge new messaging feature into MDN. Users could choose to watch only the tags they’re interested in.
My ulterior motive is to provide a motivation to migrate the MDN community to Discourse. Right now, we use mailing lists(/Google groups/newsgroups) to communicate. As things stand there’s no reason to overcome the inertia of the current system. But if people starting using Discourse to talk to their affinity groups, the need for the mailing lists might wane, until one day maybe we could do away with them.