Use case: Event post type should display certain Partner post types on front-end. However, some events may need to remove or add partners. Defining default Partners to be displayed on all Events increases ease of use for event managers.

This seems too easy to not have, as most of the functionality is already there. The only caveat to making this easy to implement would be needing to use post IDs to define default values.

See my patch that worked as of 6.3.11: