You never know who you might run into in the future. Or whom those people could influence. That's why its better not to burn bridges for the sake of feeling good about yourself.
It's a lot more obvious these days with things like LinkedIn. There are very few people that I would talk to for a potential new job who aren't either directly connected or 1 step away from many of my former and current coworkers. People talk, it's better to be remembered as the good guy and not the jerk.
This assumes the bridge isn't already burnt.
If you worked for an incompetent a-hole, chances are he'll have a negative opinion of you regardless of your exit behavior.
I've watched people bust their asses to leave their employer in the best position possible. The employer goes on to hold a negative opinion of them because they were too self-centered and stupid to understand the value that employee created. It was all wasted effort.
I noticed that every person arguing with me over this is using inflammatory language implying unprofessional and irrational behavior.
I'm sorry, so you're now casting my comments under the light of the tautology you're declaring?
The original post was about how it would have been a mistake if he had let his emotions get the better of him.
The original post was about how he stifled his criticisms and caved to the general advice to keep opinions to yourself. The exact statement was:
So I backed off and gave some general feedback—hard to find interesting projects, considering a career change, that sort of thing—without going into too much detail.
As other commenters here are pointing out, this is doing a disservice to the company and to your now-former coworkers.
Nobody is advocating "running your mouth", whatever that highly subjective and inflammatory phrase means to you.
Again: so what? What's in it for _me_?