You don't need a GOLD membership to download DLC.
Only a Live connection. (Silver can download all DLC)
The point is that some people like playing game-types with other friends.
My significant and I play Halo Reach together all the time, and now with Halo 4, we were looking forward to playing the Spartan Ops, I am the only gold account. And she doesn't play often enough to warrant a Gold membership.
The only way she can rank up now is vs me...and she has no taste for getting beat that bad...