On numerous occasions, I have started writing something, then for whatever reason decided not to post it and left the page. After dismissing the "You have started writing" popup with "Leave the page", I get on with other things. Later, I might come back to the same page... and find that what I had already written is still there, waiting for me.
Similarly, every so often I will see "draft saved" pop up on the page for a moment when I'm typing.
It seems to me that there is no harm in leaving a page with a half-written post, because it will be there when I return anyway. If I had to guess, I'd say the "Are you sure want to leave the page" was an initial solution to the problem, and the auto-saving was added later.
I don't think the popup is needed any more. And we all know what Jeff Atwood thinks of unnecessary dialogs.