Scenario - you're working on a document and it's lunchtime; you go out to lunch and when you come back you want to close the document and work on something else. You hit the X and Office asks if you want to save the document. You can't remember if you saved your latest edits, cut some pieces temporarily or what. Should you save the document and risk overwriting your good copy or should you go ahead and close without saving, maybe losing your latest work? Forget autosave, it's never on when you need it.
You can just hit the undo button to see what the last couple of things you did were and rid yourself of any uncertainty about the status of your work.