This is an idea I've seen repeated over and over. If you have something negative to say, say it to that person directly. I can disagree with this: what if you view a lot of what they do as negative, what if you fear for your personal safety, what if you know what you're saying is out of proportion, what if it's an inappropriate situation, what if there's a power imbalance (i.e. they're your employer), what if you've tried confronting them already and they're not receptive?
Perhaps it's not justified to say it to their face, then perhaps it's also not justified to say it to someone else. Must you let it fester inside?
On the other hand: how else will they have the chance to defend themselves or adjust what it is?
Why's this concept so important? What do you think of a person who just doesn't want to confront you with their views?