I think the point is that once the DL becomes a uniform document, it will become easier to check for validity. In most cases when such an "internal passport" is created, it isn't long before it starts being treated that way, and being required to move from place to place (either through blatant border crossings, or through "just checking to see if you're legit, by the way, where are you going and why?"). Many in the US are concerned that if such a uniform document is implemented here, it won't be long before the government decides to start using it to monitor folks.
What do you mean, you and I are already monitored in many ways, if we use credit cards, driver license, cell phones...this is just another layer to pull it all together and make it eeefficient for the foot patrols!