Sanguine Well that's even more confusing for users I would say. Between thanks for pointing out that, at least I can enable that setting now.
Although if we consider the two scenarios:
1. Follow the discussion I started.
2. Follow the discussion I replied.
I would say these two should be different setting because in the first one the person needs to get notified of the replies since he started the discussion for that. All the replies would be related to his discussion and he would be able to use that information.
In the second case, a person replies to a discussion maybe because he has some information to share. But he doesn't want to get notified of each and every reply that comes in that discussion since he doesn't need that information.
It would make a lot of sense to have these two setting separately.