This is my primary beef with this post. Keeping silent could punish the people left behind. I've had some bad work scenarios, and the people who said nothing in their exit interview did a great disservice to those who stayed. YMMV, though, I guess.
That was mostly my thinking. Everyone keeping shtum because "it can't help me, only hurt me..." well... if it's bad enough for you to leave, you may be the only one in a position to speak up for colleagues/friends who can't do much of anything right now. One lone voice doesn't necessarily help, but in my case, there was a pattern that was noticed after a while.