What is the benefit of this?
Files in standard locations and easier upgrades would be my vote.
I understand having a plugin for Plex, as an awful lot of FreeNAS users use it. But installing it manually, once the jail is created, is simply a matter of
pkg install plexmediaserver; sysrc plexmediaserver_enable="YES"; service plexmediaserver start
. That's it. So simple a caveman could do it. And all the plugin does is save you from typing those commands. The hard part (configuring storage, setting permissions, setting up the libraries in Plex, etc.) all has to be done manually, even with the plugin.