It doesn't matter that it's optional and you could use some other service manager. The minimal amount of stuff it does is not really that useful without registering with a service/container manager. Of course it doesn't have to be systemd and machinectl, but anything else would have to implement the same dbus interface if it wanted to work the intended way. My point is that nspawn would not have been written if it couldn't piggyback on this work that was already done. Otherwise all you have is a cgroup, some firewall rules and some mounts in a random folder, which as demonstrated here recently, can just be done with a small bash script.