While I can't answer all of your questions, my understanding of the voluntary registration at the US embassy websites in other countries is that it will allow embassy personnel to know your whereabouts in case of a natural disaster, or maybe to get a telephonic or email message to you if there is an emergency back home and someone calls the embassy to try to get in contact with you.

I've done it twice, although I probably didn't need to either time. No one from the FBI came knocking down my door after I got back. While it could be argued that this gives the government the opportunity to 'track' someone, I doubt that the state department is going to give you any guff over it.