I agree with your conclusion, but I'm not sure it applies here. Being mean is not the same thing as being persuasive, and very often they are at odds. There are usually better ways to get someone to come around to your way of thinking than making them feel bad about themselves. Making someone feel bad can actually decrease your credibility in their eyes, because it makes them want for you to be wrong.
Think about great salespeople you've known. How many use insults to motivate their prospects, and how many use flattery?
I agree with everything you're saying, but would like to add that being factual isn't the same thing as being mean, although it can be perceived that way.
"Being mean" is about HOW you present the facts. Yes, this is more or less difficult with some facts (such as getting fired), but there are ways(It's not you, it's me).
It's up to us to care enough to be nice and how we're perceived.
Think about great salespeople you've known. How many use insults to motivate their prospects, and how many use flattery?