So Facebook and Twitter should also save drafts of posts you were going to make? What if latency is non-trivial, or connection is lost? Do I also want to send ajax posts for drafts every second to ensure that as little as possible is lost? A lot of this is mitigated by a very simple

  confirm("Are you sure you want to leave?");

Yeah but for all the times when I really do want to leave, now I have to annoyingly click "Yes" in a dialog I didn't expect in the first place.

I like the localStorage solution for drafts much better.

Twitter actually does save drafts in localStorage.