Out in the open working on wheel lugs and whatnot, sure, 6-point is all you need. But you need 12-point in cramped spaces where there's not enough room to turn the wrench very far. Some of the machines I work on you have like a half inch of room to move the wrench and turn the nut/bolt. With 6-point, the head of the fastener won't be turned enough to get the socket back on it again when you position your wrench back at the starting position. You don't necessarily need a 12-point fastener to require a 12-point socket.
