When Topic A discourse is applied to Topic B, it changes the entire context and tone of the conversation. I've seen it used to make humorous posts where the OP was transparent about the fact that they would change their posts to make the person responding to them seem crazy. While being upfront about that for the lulz is fine, it can easily be abused if the OP has ulterior motives. The OP can post flamebait, some people fall for it, then the OP edits their posts to seem more grounded and reasonable, while making the people replying to them seem unreasonable.

All that said, I'm sure the mods have change logs of posts and some people would definitely notice the edits. Not to mention the Way Back Machine.

I'm sure someone smarter than me could come up with worse abuse of the post edit system, and why it wouldn't really matter too much in the end.