Joel Spolsky (of Joel on Software) said it best, I think:
This is a blockquote, but there is no styling so you would never know that…
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Q. Could you make a feature where I check a box that says “email me if somebody replies to my post?”
A. This one feature, so easy to implement and thus so tempting to programmers, is the best way to kill dead any young forum. Implement this feature and you may never get to critical mass. Philip Greenspun’s LUSENET has this feature and you can watch it sapping the life out of young discussion groups.
Why?
What happens is that people go to the group to ask a question. If you offer the “notify me” checkbox, these people will post their question, check the box, and never come back. They’ll just read the replies in their mailbox. The end.
If you eliminate the checkbox, people are left with no choice but to check back every once in a while. And while they’re checking back, they might read another post which looks interesting. And they might have something to contribute to that post. And in the critical early days when you’re trying to get the discussion group to take off, you’ve increased the “stickiness” and you’ve got more people hanging around, which helps achieve critical mass a lot quicker.
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http://www.joelonsoftware.com/articles/BuildingCommunitieswithSo.html
You could always integrate an RSS-to-email type thing, like Feedblitz, and maybe you could do something like automatically marking each topic a favorite when someone posts a reply, then also automatically check the box to email them whenever there is a post on a topic that is marked as a favorite. That would be plugin territory, I think.
I haven’t heard of anyone doing it yet, and I agree with Joel that it’s not a good idea.
My $.02
