If we make the email too aggressive, it's a substitute for visiting the site, and it kind of destroys the whole community aspect.
I think we're all in agreement that in a social/discussion forum it's OK not to have email notifications, because in a social/discussion forum that's would be, well, antisocial.
In tech support forums, the question is whether you prefer to build up a group of people who hang out, thus making benefit #2 more likely to happen, or whether you prefer to make sure that customers get prompt replies to their posts, at the cost of sacrificing benefit #2.
Email is for the less urgent stuff where you're busy and don't have time to check the site.
Part of the implicit contract at SO is that you "pay it forward" and help others answer their questions while you're getting an answer to your own question.