I'm calling this a bug because I consider it a bug (an unintended consequence) in the UI.
It is currently possible for members of a group to have email delivery set to Special Notices and advanced preferences set to Following Only. My group has had several new members recently set themselves up that way. I'm sure they think that this means they will get emails from topics they follow. But they won't. They will *only* get special notices they follow, which means that they will get zero messages (under normal circumstances). Special Notices aren't really the kinds of things people will follow. But more to the point, allowing someone to be both Special Notices and Following Only makes them think that they will get emails from (normal, everyday) topics they follow, and that's not the case.
I think the UI for advanced preferences has to be redesigned to disallow the combination of Special Notices and Following Only, and to allow Following Only only if the member is also subscribed via Individual Messages, Digest, or Daily Summary.
Messages are the sole opinion of the author, especially the fishy ones.