I have found the thread notifications very useful for me . I think there is no need to notify a user by email, when he replies to the thread himself, which he has marked to get notified of new replies.
it would just add the use of bandwith the fact that users get to reply threads all the time. each of us will definitely bombard with alerts, its like using a xenforo which can make bitcointalk loads very slow. I don't think its a good thing to implement unless there are just few users here.