The truth is, there is good and bad in every artistic work. Some will see all positive, others all negative, but most will court the gray places in your endeavors. The ones unashamed of the shades between your blinding white and debilitating black are your gem-like reviewers. Those are the people who really think about what you've TRIED to accomplish, what you DID, and where you FAILED. They're immeasurable.
But we only want to hear from people with balanced feedback BEFORE our work hits shelves or cyberspace. Once it's there, all we want is to see our radiance, amplified and reflected, towards millions of other hallucinat-- uh, devoted... fans.
I get it, I do. It hurts to hear something you've worked on for so long didn't turn out as clever or unique as you'd hoped. And we need the excellent reviews to buoy us when people post incredibly unfair comments about your book (which they somehow "forgot" to read) before typing their vitriol.
We still shouldn't clamor for honesty, however, unless we're acting out another bit of absolute fabrication. In that case, tell your readers to be as open as they like.
Do you ask for honesty without wanting it? Have you ever given a sincere review only to be slammed for it?