I’m not aware of any assertion that primary-group authority provides any easing of user profile management, though I don’t know that it won’t. I am only aware of the potential for better performance of authority checking. (Such performance can be significant in some cases. You need to evaluate it for your particular environment where group authority is even meaningful.)
See Group ownership of objects for your i5/OS version/release for any specifics. Review the paragraphs marked as ‘Primary group for an object’ in particular.