Depends a lot on the context behind that leaving. There are absolutely situations where it can be warranted - abuse (including verbal), for example. Politeness is the default state of affairs, but depending on how you behave, you might lose that privilege.

In any case where a 'fuck you' is warranted deadpan silence works better.

Sometimes the company deserves it.

you usually don’t want to burn bridges, true. However don’t ever let anyone treat you like shit. Walk away. Burn that bridge to the ground.

Like the time where one of the seniors was clearly an asshole and was right on the border of racism. My manager wouldn’t do anything to jeopardize his promotion. He was clearly a bully.

Fuck that bullshit. I walked away as soon as I got my green card.

Sometimes the employee deserves to vent out his anger. He needs closure. He needs that satisfaction that after years of exploitation and humiliation he had the final word.

"Don't burn your bridges" is a good advice only when ghosts of your past are not going to cross the bridge and attack you in future. In many cases you should set the bridge on fire and destroy it till the last brick.

My point is that losing your composure is undignified, let them be the angry ones, they should understand that you have lost nothing and it should eat away at them. Giving a 'fuck you' is in a way providing closure for them.

So, basically, don't do it to hurt them more?

What if you want to provide a closure for yourself, and don't care about them?

Exactly. You should be good enough at what you do that you are never the one in need of closure.

Closure is an emotional thing that is not necessarily (in fact, I'd say not usually) connected to "being good enough" in any meaningful sense. If anything, it can be the opposite - when you realize that you have been treated in a way that is far beneath the value that you were providing, the realization of that fact can very well lead to a "fuck you".

Totally agree. It might feel good to drop the f-bomb on them but it's never in your best interest.

A good adult thing to do is to start recognising that different people have different ways of dealing with situations. A good adult thing to do is to start understanding that there is no one absolute answer to almost any question one could ask - including, but not limited to, the question: "Is it appropriate to tell someone 'fuck you' on the way out the door?".

Adults realize that actions have consequences. Saying fuck you to someone might make me feel better in the moment, but what would it give me in the long run? I would be giving others the chance to view me as childish, unprofessional, and uncivil. Where is the gain in that?

If you were childish, unprofessional, and uncivil, it would be the honest thing to do. Not everything has to be done for personal gain.

On a related note, I'd much rather be told an honest "fuck you" than a dishonest "it was good working with you". The former is an opportunity for me to reflect.

Not necessarily. Know a girl who said fuck off to her boss and walked out of the cabin because the boss wanted sexual favors and tried to force himself on her. Honestly even punch in the face would have been a very adult response. So it depends on the situation.