I value being viewed as someone with integrity. If I'm using this exit interview as a chance to be honest with them for the first time, then it's my fault. By the time I made it to the exit interview, everyone in the room knows 100% why I am leaving. I have already tried to change the situation, and it didn't work.
If a girl asked you if she was fat (and she was) are you going to tell her that she is fat? Or are you going to lie? Everyone in the room knows that she's fat. No, you're not. You're not going to ruin her day. That's how I view an exit interview. We all know the truth and If you're going to make me say it on record on my way out, the message will make it back to management and I will look bad.
I guess this post makes me look like a miserable POS, but it's really the opposite. I've had great experiences working for great companies. At a recent job at Big4 firm, the CEO of the firm sent out my resignation letter to the entire company because of how great it was. It was a positive email, with no negatives. There was some constructive criticism (very, very minor criticism), but it didn't make it to my good bye letter or exit interview. I kept it positive and in the end it worked out great.