Hi,

I do not have a straight answer for your question.  I tried to put my understanding of Privileges here. 

1. When we are using the privileges to control the UI like Navigations and Sections, running Report Definition, the privilege must be present in the hierarchy of the applies to class of the rule. Here we can not control the access by using privilege from other classes. 
2. But to control the execution of few rules like activity, flow action, Summary view, list view, Parse Structured, Corr rules etc; we will have option to give (className, privName) combination explicitly. In this case we can give any class name.
3. Other important part is to granting the privilege to a user through Access Role Object. For granting privilege on a class (through RARO instance), it must be present in the class  hierarchy.

Considering above three combinations, I think creating privilege in the current class or anywhere in the hierarchy will solve all the above three use cases. If we have a privilege created in the system and we want to use the same in scenario like #2 above , we don't need to create new privilege in the class hierarchy, instead we can use the existing one with class & Privilege combination (as long as the access restriction purpose of the created privilege includes current scenario as well). 

Comments are welcome.

Regards,

Santhosh