This is a matter of documenting already-made decisions so new and old developers can be aware of that
- Always install them
- Always install them. *NO* use flag
- Always install them if they are provided upstream. At maintainers discretion if there is no unit file upstream. If a maintainer wants to add support but he can't test the changes, CC the systemd team. Users/devs can opt-out installing these files using INSTALL_MASK.
I guess, a note telling that, in general, USE flags for controlling installation of one kind of file is discouraged (for example, I think we now opt for not controlling bash-completion files installations using USE flags)
What about zsh completions? We install them with a use flag in several
places in the tree.