My feedback provided a written-down justification for investing in some much-needed improvements to the way they built software. A few things tangibly improved as a direct result, and that is valuable to me.
I suppose the flip side is that people will say I should be eliciting that feedback all the time, and I do, but there's a difference between giving feedback to your superior and your former superior.
At that point you have a risk/reward trade off, and an absolute "NO!" rule that's based on the generally correct perception there's no reward is less relevant. BTW, did you give this feedback to HR or to a manager type? Headhunter Corcodilos is particularly inveighing against the former, although a lot of his points apply in all circumstances. And he does allow for doing this with the right person, with a strong recommendation of waiting a couple weeks after you're gone.
To finish, there's always the question of "Why will they listen this time?", but, hey, I was born, raised and have retired to Missouri, where we're famous for our mules. And I was told the first rule in interacting with them is to get their attention with a sturdy piece of wood. I'm sure in some situations, especially the ones where you know all the players, that can actually work, even if it takes something even more drastic like resigning.
Hmmm, you might even point out that they should try playing this game before they lose the next person....