Well, we were originally thinking about using it as a Mumble or voice chat, not really in the context of IRC or text chat. Looking at it as just voice chat, it seemed like it'd probably end up being a waste of time that wouldn't take off. But looking at it more recently as like IRC, I could see it being pretty active if it gets off to a good start.
In terms of downsides, I'm (Jeremy as well) naturally never a fan of things off-site if we can avoid it. While IRC is still technically on another server, it's so old that we know it's never going to disappear and we are also able to embed a chat right into the site directly. As far as I know, Discord doesn't have any proper embed option for the actual chat beyond a small widget to join, meaning that we have to send users off to a completely external site for the chat. Arguably, that's actually less user-friendly than IRC. Discord is also a fairly new service that appears to be solely run off investment right now without any real business plan in action, so I have my doubts as to its longevity. I'm not sure we really have the moderator capacity right now at this moment for a very active chat either. To be frank, moderating IRC when it was active back in the day could be a real pain in the butt sometimes.
Also in regards to other existing servers, I've been aware of a few although not quite sure how large they are. Regardless, people are of course always free to have their own groups. An official one would be more for the purpose of what IRC was once upon a time -- a general open chat for everyone.