You can change your vote for a question (or answer) after it has been edited. Quite often at least questions improve soon (because the questioner is told that the question is bad and what information is missing).
I caught myself thinking after downvoting a question or answer: Will I notice a possible improvement of this post and thus revoke my downvote?
Thus I thought it would be nice to be informed of edits to a post which one has downvoted. I could understand that people who vote very often do not want to get many notifications (though that would contradict the idea of the vote change possibility after an edit) but you could be asked everytime you downvote: "Do you want to be notified of edits to this post?"
Additional thought: If a downvote is revoked then the author of the post should not get back both reputation points but just one. I think there should be a difference between those who ask good questions and those who just repair their bad questions after causing the trouble of having to tell them to. (There already is an implicit penalty, of course: You don't get upvoted while the post is bad and most readers will not come back to it later.)