New users sometimes leave a question, check the "Notify [me] daily of any new answers", and then don't return to the site, knowing that they will get a notification when someone replies to their question.
Is this mail also sent when the reply is a comment but not an answer? If a question needs some clarification it is the custom that you don't give an "answer" and work from there, but comment directly on the question and ask for edits. If no notification is sent to the new user, they might completely forget about the site, and the question remains unanswerable.
I modified the query that Jeff used to auto-delete old questions, and I noticed that 370 comments were posted after the OP last accessed the site. Many of them are comments asking to improve the question, but these messages never got to the OP because they think they will get a notification when something new is posted.
I think this is especially important for the newer Stack Exchange sites that are still growing: new users don't always know how the system works, but they can be potential goldmines once they become active users. I don't want to lose them because they think no-one replied to their question.