Thanks for your responses, everybody Paul, I agree about that oddity of the optional subscription settings.
I guess the questions to me are: what should the average user expect? And what counts as annoying? Let's say you are an undergrad in a course with 15 other students and your course has a subject-based discussion. Is it annoying to get emails from other students who participate in the discussion or is that just actually participating?
Or, if, like you mentioned, Derek, a client says that they found four emails to 'inundate' them, is it possible something else has gone wrong (like your client's attitude)?
I have been enrolled in ALT's MOOC where over a thousand students were asked to introduce themselves and share their thoughts. Every single personal introduction was sent to me. Now, that, to me, is a good example of using subscription settings badly.