If programs aren't allowed to add themselves to unix.hosts, then it will be far less useful.
People using Syncthing aren't going to want to edit some textual configuration file in the process of setting it up. It needs to Just Work, as it does now with TCP. A unix.hosts file that can only be hand-edited does not Just Work.
/etc/hosts is generally considered useless, by the way. It was replaced long ago by something that Just Works, namely DNS.