If I declare Crocker's Rules and you write something rude in a reply to me, other LW readers still see it. So even if I am perfectly okay with it (and I shouldn't have declared CR otherwise), you might lose some status in the eyes of the observers who don't properly evaluate the context of your reply. If you send me a private message, we get rid of the observers. Unless I play dirty and later show the private message to someone else. Anonymous feedback would prevent me from doing so. But yes, for 99% of cases, sending private message would be enough, anonymization is not needed. And we already have that option here.