Does the truth hurt? Yes. It can. But it is the truth.
No, it's an opinion. The quality of your essay, the size of my bum, the cut of that dress or the taste of that meal are nothing but individual opinions on what is probably a non-absolute phenomenon.
There is no abiding truth about the quality of an essay - submit one to two different tutors and see what happens. But taking an opinion an running away with it as Truth(tm) is a fast track to thinking that people who disagree with that opinion are either stupid or misguided.
People use honesty as a cover up for laziness or cowardice. For example, I absolutely loathe one of my friends' girlfriend. I mean, I seriously cannot abide the woman. I have two choices - be upstanding and honest and tell him, thus putting him on the spot: landing the ball in his court as far as making time for meeting without her present is concerned; or make the effort and suffer quietly for the sake of our friendship (and you've not known suffering until you've spent an evening in that woman's company, let me tell you). After all, my opinions are not the guiding light of his romantic life (thank God).
It's not just white lies, it's not just condescending benevolence that we're talking about - spouting every old thing that comes into one's head is simply infantile selfishness. Labeling our own narrow (and they're always narrow, the limitations of the human mind being what they are) opinions immortal truths and imposing them on others is an extremely adolescent thing to do, and is likely to embarrass us in later life, when circumstances and opinions change and our "truths" no longer conform to our reality.