That said, I'd recommend simply refusing the exit interview. That's 20 minutes you're better spending elsewhere.
I believe it is also called 'business correctness'. It's like a lifestyle disease. Not exactly a killer on its own, but inviting all sorts of other troubles into work and business. Anyway, am not convinced refusing is in any sense productive/curative of the disease. May be if you refuse and then write a blog about it, but that would be major war and might cause career suicide. So i would rather give feedback and level of politeness, well that's a personal choice.
Refusing doesn't cure anything, but it highlights the silliness of the exit interview, and adds 20 minutes to your life. You are right that sharing your honest opinion with others (which seems to happen over beer, more often than not) is far more curative. Not participating is more like an act of passive resistance protesting the hypocrisy of it.
Sorry, are you complaining because people are denied an avenue by which to slander others? The utility isn't ruined. You just have to look at it a different way. "Good" people will probably have a good reputation because sharing praise is almost always safe. Bad people will probably have a vague reputation. Doing things this way lets people be rewarded for good behavior, while people who have made mistakes get a chance to learn from their mistakes and "start over".
So true. So very, very true.