Microsoft documentation on this isn't clear, but built-in local groups like Account Operators, Server Operators, and others found in the Builtin container of Active Directory are legacy groups that are basically only there to maintain backward compatibility with Windows NT.
If you want to grant users rights to perform certain tasks like create new accounts, reset passwords, and so on, avoid using these built-in groups and use Active Directory delegation instead. Delegation gives you greater control over which groups of users you can assign to perform different kinds of admin-level tasks, and it's easy to use as well, just right-click on an OU and select Delegate Control and a wizard opens to walk you through the process.
Cheers,
Mitch Tulloch
MVP Windows Server
http://www.mtit.com
