I feel like a lot of mindshare moved over to XBMC. Kodi has a better user interface, but I think MythTV had a better backend.
It was always somewhat challenging to get everything working perfectly, but the ability to set up recordings on one TV (or remotely by web) and watch them on another TV was/is fantastic.
Playing music was always wierd - you needed to set up nfs mounts from the music server to play the files on your remote screens - it would make a lot of sense for them to be streamed automatically from the mythtv backend. But I get the idea that maybe some quite old protocols are in use and it would be a lot of work to change them.
Long Live MythTV/ MythXubuntu