Companies that really do value honest, constructive, confidential, consequence-free criticism (the best kind in some cases) will value that feedback most from their current employees. But, in doing so, eliminate the need for the soon ex-employees to feel the need to dump reality on management.
Why? Not because I thought anything would change (it didn't - it got worse and most of my friends have now left too). I did it because I knew what I said would get out, and I knew when I ran into my past colleagues in the future (it's a small industry) that they'd have good laugh about it all and only have respect for me for sticking to my principles and never backing down.
All the best people I've worked with have been straight-shooters and I admire that and look for it in the people I interview. Every position I've ever taken on has been through word of mouth, including that position. You can use this knowledge to your advantage. I think this blog post is overly conservative.
I think the blog article is trying to tell newer/naive employees that, when asked by a soon-to-be former employer to provide an honest exit interview, the whole premise is a charade. How we handle that charade is still up to us, though.
When I left my last job (Fortune 500 co), the HR exit interview was the usual crap and a total waste of time. But the Director of software engineering pulled me into his office for his own exit interview because, in his words, "I know you can be completely honest now." He genuinely wanted to know from one of the "troops" how things were going and he wanted the data unfiltered by HR policies.
That's the kind of exit interview that makes sense. Not surprisingly for a company with that kind of management, I honestly didn't have anything negative to say about them.
The real question is what they do with the feedback (whether it's from an exit interview, or during yearly reviews, etc).
The fact a person feels the need to "vent" at their exit interview shows that the company has not been seeking that kind of feedback during the employment duration.