Acceptable reason for emailing an hour before a paper is due and asking for an extension: My roommate OD'd last night, I was the one who found him and called the paramedics, he almost died, he only regained consciousness an hour ago, I have police paperwork to verify. I could have done without some of the graphic details, but I don't expect someone who's just been through an experience like that to have a fully functioning courtesy filter.
Unacceptable reason for asking for an extension to an extension: I have another paper due on the extension deadline and I don't want to have to ask for an extension on that paper too. This is crossing the line between an understandable request due to circumstances beyond your control to the undergradese "please rearrange your life to suit mine".
I'm sorely tempted to institute a "turn in what you've got on the deadline or turn in nothing at all" policy when I actually have control of the class you teach. If I'm going to feel like the bad guy (for not being completely flexible), I might as well be one.