Should be part of wordpress core

Adds the possibility to define n:n relations between post-types and/or users. You get nice admin backend additions and the api to fetch and display the relations in the frontend.

Neat extra: You can also define meta-fields for those connections.

Example: Create custom post types for employees and departments. Using the plugins API, create a connection between the post types (“is-part-of”) and add a meta field (checkbox) called “head-of-department”.

From now on, you can connect employees to departments and define an employee as head of a chosen department.

Imho, that plugin offers functionality that (amongst others) should have been part of the custom-post-types infrastructure in the first place.