As someone who managed a gaming guild since 2005, I have seen many iterations. In the beginning it was all Skype, then we switched to Ventrilo and PhpBB, then many went to FB for text, then to Teamspeak for voice, then to Slack for text again, and now Discord for both text and voice.
The needs of the community didn't really change. They wanted persistent chat for text and images, put up announcements to guide new players, have votes/elections, and use team voice chats during play sessions. Each combination above solved those problems, each with its own unique benefits and drawbacks. Mumble's use case is exclusively on the team voice chats but it lacks any direct support for anything else which is likely why we never used it, even after I provided a server for free.
Nowadays I use my Mumble server for cooperative remote work, a job which it works perfectly for.