I think a 'no CC' message in a signature block looks unfriendly, even
stupid. Like the legal notices about not reading wrongly delivered
email. I would not want to create an environment in which any help
giver felt an urgent need to do such.
As a matter of fact, the current rule is helpful to me in assessing
the advice that I get. If I get a CC, I think this guy isn't a real
DD --- I wonder if he knows what he's talking about.
But if I violate the rules of list etiquette, I prefer to receive an
email addressed directly to me rather than to be publicly shamed on
the list.