You wouldn't have to write service/unit files for one off commands. The release notes mention that you can use 'systemd-run' to start these[1].
[1] - https://github.com/systemd/systemd/blob/master/NEWS#L29
[1] - https://github.com/systemd/systemd/blob/master/NEWS#L29
Sounds like a lot of pitfalls with these new defaults. Wonder how many background tasks will be ruined before somebody patches screen to automatically issue all these new systemd commands.
[1] - https://wiki.archlinux.org/index.php/Systemd/User#Automatic_...