Exclusion. In order for a group to be unique they have to exclude people; set a limit to the outside world of who belongs to it and who doesn't. Otherwise there's nothing special about it. This is particularly important if that group is desirable to be a part of. The current rule of one drop will persist until the socio-economic conditions change. If being 'white' is no longer associated with being rich and privialged, then people will stop asking to be a part of it. my 2c on it