In some case, if a FK refers to a table containing 1:1 link, the form would be able to show the information of any field of any extension of this table. Very often with the #__users table.
Example : One of your forms instance a Foreign Key to an account. This account link with #__users table in 1:1.
Then in your form you could show the 'username' property of the users table.
Only for this exception (1:1), Cook should not limit the FK combo to the first root level only.