Anger is a form of communication. If anything, OP would be helping to send a message that if you push people far enough they will act out, maybe they shouldn’t be pushing other people quite that hard in the future. Boundaries are being crossed, and badly.
It signals it in such a way that the employer will attribute your decision to your lack of professionalism and not to their own actions.
> I was leaving at a critical juncture and the company begged for me to stay at least for 3 months to complete an important project. I said "fuck you" and left.
Does the "fuck you" here signal unambiguously that this person is outraged at the way they've been treated? It could just as easily signal that this is an asshole who will screw you over when the opportunity arises.
Something along the lines of "I'm sorry but I can't continue to work under the conditions you've put me through the last 9 months" would be a much clearer signal. Maybe throwing in some swear words would improve the message, but a "fuck you" on its own leaves unnecessary ambiguity as to who the asshole is. Perhaps I'm overly concerned about my reputation, but I'd rather not add room for people to side with the person who mistreated me.
If you set your end of our bridge on fire, I'm nuking my end from orbit. Why? Because screw you. If you don't need me, I sure as heck don't need you.
Acting on feelings is usually good for the mental health, even if it seems irrational or inefficient.
Don't knock it until you try it.
I'm still not sure what your point is. Are you saying that the "fuck you" is the best long-term solution once you factor in your mental health? Or are you saying that the short-term mental health benefits of the "fuck you" are enough to justify the long-term detriments?