Joel on Software has a good take on this - http://www.joelonsoftware.com/articles/BuildingCommunitieswithSo.html
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.