Summary

Project admins are able to configure their Projects Roles and choose groups which they want to be part of a role. They are able to choose the group jira-users as well as other groups, some of which are groups which all users in JIRA are members of it.

Allowing project admins to choose jira-users might be useful for some, but for others this might be a big problem especially when they have their Notification Scheme configured to send notifications to entire Project Roles.

Project Admin unknowingly adds this group to the role causing all users to receive unwanted notifications.

Another Use Case : 

Allowing project admins to choose only groups and not individual users while adding to project roles. This way all the roles related to a project can be controlled through groups and hence managed in an easier way.

Suggestion

Make it configurable to hide certain groups or users so that Project Admins will not be able to choose them, or maybe only users with JIRA System Administrators permission can choose them.