I agree, this is useless. Just go back to that topic and see how many people replied. It would be easier this way, but it's only like 5 seconds of extra work.
This would be really annoying, some people start topics like "The Lyric Game" that get upwards of 600 posts on it. They would be swamped by E-mails, and is it really causing you so much grief to have a quick skim through whatever forum for your topic? That's what I do.
Actually just to clear things up how it usually works is if you post something and get a reply it sends one e-mail and never again until you post again. Won't send 600 but just 1 everything you post and get a reply after your post.
I think it's an awesome idea if you have the option to disable it! It would be great for me cause I always lose my topics and when I can find them I often check.
note: if anyone knows where the topic 'under 13yr members' has gone post a link to it on my profile pls!