Can you point me toward some other clients that are full nodes which the protocol requires?
Nobody needs to be forced to do anything or obey anyone. It's the whole point of Bitcoin: Decentralization.
By "requires" I meant needed to continue to function properly.
I never mentioned authority or force. My question was genuine.
I use several alternative clients because of the fantastic features they offer, yet I still run the Satoshi client because, as far as I know, it is the only "full node".
Was your complaint that wikipedia doesn't list alternative clients? (I only browsed the page.)
According to Ufasoft, their alternative bitcoin client is also a full node! Check the alternative clients subforum.