Posts are made for various reasons. To give incomplete but reasonable categories.
1.to educate
2.to be educated
This doesn't undermine many other reasons of making a post.
Let's take no. 2 above for example. When a post is made with the expectation of being helped by others through their replies, it makes sense for their to be a notification of replies to that topic.
(except the starter did not on it)
And I think it should apply to the reply section also (where it becomes really optional to get notifications on reply made on other people's topic).
And then notification when quoted by others
I don't know how best to communicate my points. But I am just rolling around notifications (anywhere and anyway that notification appears necessary)
Altogether I believe it will help.