I would rank that statement right up with the statement that "my donation is for my time companionship only." Nice gesture, but its not worth the paper (okay, computer screen) it is written on.
Yeah, pointless in the technical sense that you could actually weed out people. It's so dang prevalent with porn and agency websites and even this website when we log in, I was thinking maybe the "powers that be" on the Internet require it. Wouldn't want to get in trouble over a stupid "technicality"
Not sure of the "legality" of not having it but I do think it provides the site with some plausible deniability that they are not letting in underage people. I don't think it provides any protection for you however. Like you said, it is pretty much standard issue on any "adult" site. I'm sure one of our more "legal-oriented" members will be able to clarify.
In some areas, yes you have to have it. Best pratices would be to have it to avoid issues and warn people who might be at your site by accident and not want to view the content.