I just came across this topic when i was searching for different ideas on "using multiple roles for similar requirements".
I think that, a feature as a container for related stories helps in prioritizing the requirements because stakeholders usually tell their needs as dependent stories. In a recent project, the customer told me as follows
A member can send messages to the admin
Admin can send messages to all members
Members can send messages to each other
When i see these requirements, i know that, we should implement a system to enable people to send a message and we should add checks to allow whom to do what.
And also i know that these requirements may have some other implicit requirements such as reading the messages that came, arranging them , may be setting as spam and etc.
So i try to rephrase these requirements as
As a member or admin, i can send messages to other people.
As a member or admin, i can read messages that were sent to me.
And as the acceptance criteria, i state in detail who can send to who.
Then i call all these things as "Private Messaging" feature, so that, at some time later, if the client decides that it is an extra cost, he can say "Just drop the private messaging thing" and i can remove all of them from the backlog.