Wondering the same! I added my DF and DD as AU today on my CO QS1.
Adding an AU that helps your scoring is not a free lunch.
It automatically means that your score is no longer representative of only of your own history.
Creditors know and understand that a score based in part on inclusion of the credit history of another is not representative of your own risk analysis. If they do a manual review, which is common with higher amounts of credit, and see the presence of an AU, they may choose to discount the value of your score in their evaluation. Some lendors may even require removal of any AUs as part of their standard underwriting process, enabling them to then obtain a "real" score.
If building or rebuilding and only applying for low amounts of credit, the creditor may not invest the time and expense in doing a complete manual review, and thus the presence of an AU may not be an issue.
However, as you move up the credit chain, presence of an AU may raise concerns by the creditor.