Plate readers are expensive and most departments don't have one, and if they do, they have maybe one. The 70% number doesn't mean 70% of cops have a plate reader. Our department had over 500 cops, we had one plate reader.
For it to be useful, the cop has to be driving around in traffic. If there is low traffic, it's useless so they usually put it on cop cars that are going to be in heavy traffic and often.
If you have five pounds of weed in your car, a dead body in the back and your vehicle has bald tires, the plate reader doesn't know that.
It's a tool to find stolen vehicles and wanted persons.