Let me explain a bit : One day on IRC we were speaking about Global ID, and Timbo explained that it would cost quite a bit of money to set this kind of thing. Since players dont like to spend money, you will never see a Global ID system.
And yes it could be useful to prevent griefers, since there wouldn t be any ID theft, so you could screen and send it to an admin of the server, who would be able to ban him when he comes back. Right now, you have no idea who is playing with which nick.