Scenario: a corporate instance of Phabricator in which a guest user have to be involved for a specific task
Problem: the guest user should not be able to see all the rest of the corporate things but only what he has to work on
It is true that is always possible to restrict visibility of things, but instead of always forcing to use a different than "All users" visibility (that is the nice default), another possibility would be to have a specific user flag that limit his/her visibility by default.
There can be a (new) flag specifying the tags of what the custom user can see even if the objects policies allow more, and if this tags are set to none the user will see just the objects that will have a custom policy explicitly allowing him/her.