I get the point that people can come back and check threads if they are bothered, but often it's not that simple.
Without reply notification, I might post a question. I come back and check several times over the next few days and no response. In my mind I've pushed my question to the back burner. I've got other stuff to do.
If someone is kind enough to post a reply I may not see it for days, until I come back. The responder has given up on me, cos they got no response or acknowledgement. The cycle goes on with days, even weeks between responses.
Another use case is someone specifically registers here to post a question, it's not obvious that they must come back and check. They are only a casual user. People are busy and need a nudge. Does everyone know about looking at their profile ?
With notifications this problem goes away. Admittedly there are some technical issues with bounced emails etc, but that's a technical issue, not a usability issue.
As I said earlier, I think it would help greatly to make WP a better product.
As an example see http://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/wordpress-database-error/#post-51754
Rhetorical question, but what are the odds that the OP will come back after two weeks and realize there is a workaround ?
Please add email notifications, even with a default unchecked 'notify me' control, would be a huge improvement.