Please correct my understanding if it's wrong, but it seems that Plexamp's use of MPD (Music Player Daemon) is embedded into Plexamp itself, and not a part of Plex Media Server. This means that, in order to get MPD functionality, you can only do so through Plexamp, and not through an MPD-compliant client.
I suggest/request moving MPD into Plex Media Server itself to make it a first-class feature.
The benefit of this would be to separate server/client concerns out of Plexamp, allowing it to be purely a client in and of itself. Additionally, if MPD on Plex Media Server is indeed a fully-compliant implementation of MPD, people can choose which client they want to use with it, thus increasing the value of PMS as a platform.