The idea would be so that you could fix a simple grammatical error without having it say "edited by so-and-so" which I find annoying sometimes. Anyone who is copying someone else's answer will almost surely be adding/deleting more than 10 letters, and it could be set so that only the first time this is done it is not siginified, so that a user couldn't just edit their post a whole bunch to copy someone elses by editing it 10 letters at a time. I realise that is the purpose of the "edited by blah-blah" not showing up when a post is edited immediatlely after being posted but more often than not grammatical errors are not noticed until a little while later.