SMJ, speaking for myself, I consider feedback a gift. Even when it comes in a paper bag flaming on the front porch (figuratively, of course), it is saying something of value if I can only hear it. If nothing else it will reinforce my sense of humor, but in many cases there is a nugget of what I needed to hear but others, out of politeness, were withholding from me, and that's why feedback is a gift.
Ever heard of Johari's Window? It's a very cool psychological concept that I learned about from Kim Peterson, author of the PMTS and PSIA-RM guest centered teaching models. http://www.cps.usfca.edu/324sh/johari.htm
We all need feedback from others because we have a Blind Spot -- stuff about ourselves that others know (and aren't telling us) and we ourselves do not know. The piece of green junk between your teeth is an example. When you see it in the mirror you're aghast, "Gosh, I wish someone had told me."
Feedback, it's all good.
This probably does not address your point about politeness. I agree that one gets more flies with honey than vinegar, but I also like Bill Murray's quip, "Who wants flies?" Sometimes attempts at writing humor come across as rude, crude, or cruel--the light touch that is so critical takes a long time to develop and sometimes never comes. I attribute that to some of the harder-edged writing that people do around here. I like to keep my pulse slow and steady, so I tend to perceive and not judge what they say.