People change. People change even faster when you give them feedback.
It depends what you mean by "feedback". If "feedback" is a polite, respectful reply explaining the mistake then, yes, it is something the other party can learn from. If "feedback" is a downvote chances are it is only going to hurt the other party and possibly make her even more entrenched in her position out of anger. When you argue respectfully, the other party can admit her mistake with small emotional cost. If you call her an idiot, admitting the mistake will become much more difficult for her (since it will become emotionally equivalent to admitting being an idiot).
I downvote things I don't want to see from people I like and respect the same way I would frown at a friend if they did something I didn't want them to do.
First, you can allow yourself more with friends because they are friends. Second, a downvote is a sort-of public humiliation, it is much worse than a frown. Imagine that a person you would like and respect makes one of her first comments on the forum and gets downvoted. She might become so upset she won't return here again.
There are several points here that seem entangled, but I'll try listing them separately.
First, it is a desirable quality to be able to work out what one did wrong from minimal evidence, or repeated experimentation.
Second, it seems to me that rationality is strengthened by the ability to joyfully accept contradictions and corrections. A view that sees a downvote as a sort-of public humiliation is probably too sensitive.
Third, politeness is costly, in several ways. Most relevant to the others is the time cost of writing a reply. It often takes much longer to...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Thread started before the end of the last thread to ecourage Monday as first day.