CMP is supposed to be about shooting, and encouraging the discipline of individual rifle marksmanship. Joining an organized club is consistent with that goal. They are VERY flexible though, hence you can cough up 25 bucks to join the GCA, even though you won't learn marksmanship skills that way.
You get a very informative magazine, with pretty color photos. Further, membership lists are updated weekly to the CMP (you can opt out if you wish). That way, you don't have to send proof of your GCA membership. Just write "GCA- see database" on your order form.
I belong to both GCA and my State Rifle and Pistol Club, so I'm doubly covered.
But no one is trying to put something over on you. You don't have to join. You don't have to have government subsidized Main Battle Rifles and cases of ball ammunition delivered directly to your front door.
What I love about America: Public Libraries and the CMP! (among other things: sorry you don't Get It, Ms. Obama.)
(Hey Rugerlvr, where are you getting your repro bayonet?)