If license plate readers continued to proliferate without restriction and the DEA held license plate reader data for extended periods the agency would soon possess a detailed and invasive depiction of people’s lives, the ACLU said, especially if combined with other surveillance data such as bulk phone records or information gleaned by the US Marshals Service using aircraft that mimic cellphone towers.
"Data-mining the information, an unproven law enforcement technique that the DEA has begun to use here, only exacerbates these concerns, potentially tagging people as criminals without due process," the ACLU warned.
That last part is what I'm most worried about. There have been several stories over the last couple years of "law enforcement" agencies badly abusing the data they have, leading to over-policing, false arrests, and cries of "pre-crime". Some of this is growing pains, while society as a whole learns how to more properly use advanced scientific methods. Some of this we might also choose to avoid, on philosophical grounds, if only we knew what was happening.