I don't see a big problem with this. But police need to start enforcing the rules about visible license plates more consistently or it won't be so useful.

Obviously it would be easy for the to expand the usage of a license plate reader to a wider sense of circumstances, but it would not be easy to expand it to something like facial recognition--hard to recognize faces from behind. In the long run though, I have to think things are going to move in that direction--the police [[and not just in Detroit or Michigan) are extremely bad at solving crimes and they need all the help they can get.