In general we have multiple role in AD, there will be a minimum of 2 DC (Primary and child) to avoid single point of failure.
Just a little detail, you should definitely have at least two domain controllers to get redundancy and they should be in the same domain - which is not really called primary and child. If selecting "child" in dcpromo then it will be another subdomain which is not the wished configuration here. The two DCs should be hosting the same domain database where there is not really a primary one (with some exceptions..) 
Anyway, I totally agree that a minimum Active Directory setup should have at least two DCs.