I actually like the idea of tagging people. True, it does open the door for more invasive surveillance of people, but if it were properly controlled, it could be a great benefit to society.
Plus, it's be convenient. No more carrying wallets, credit cards, passport, house keys.
I would have no real problem with the gov't knowing where I am, I don't do anything illegal enough for them to come after me. Also, as Kotario says, you can (almost always) be tracked, this would just make it more efficient. Hell, it'd even save money on policing costs, so it'd pay for itself.
Then there is the potential for medical records (etc) to be put on it. Allergic to penicillin? Haemopheliac? Organ donor? Jehovah's Witness? (they are often against receiving blood donation). All this could be on a chip in your arm/leg/head preventing medical cock ups.
And I'm just talking about an information/tracking chip here, not a "useful" implant.