I think all of what you say here is true, but at the same time, I don't consider it sufficient justification to have a policy of always adding an EDIT marker. The simple truth, for me, is that when you get down to really unlikely edge-cases regarding possible misunderstandings or whatever, I just don't care that much. At least not in most cases.

So I suppose you could say my policy is a sort of subjective, probability weighted policy, based on the expected probability of my edit leading to A. a misunderstanding and B. that misunderstanding being one I would care about.