Of course, I don't have a particularly large sample here: just three companies. If I had followed the advice in this article, however, I'd never know what might have happened if I had actually spoken my mind, and I think that's a tremendous waste. If you've already taken the initiative to leave a job, why let yourself be so paralyzed by fear that you won't speak your mind in the exit interview, the interview set up by the company to attempt to capture the reasons for your departure?
I don't believe in corporate loyalty, and I don't believe that you owe a former employer much, but I'll say this: If you want to be viewed with any integrity, and as a person of merit and a person that your coworkers - and even your former employer - would consider worth maintaining a relationship with in the future, you owe it to them to be honest in an exit interview. This doesn't mean you should take it as an opportunity to vent and say horrible things just because you can, but you should be honest. If they opt not to act on your feedback, that's their fault, not yours, but if you fail to give them actionable feedback that's your fault.