In my state (Illinois), a police officer can retrieve the digital photo that was taken at the time the license was issued. A simple radio check can also verify the other information on the license.

In reality, if you know your license number you don't even really NEED to carry the card. The app just just an easier way to carry that number around. I've known people that have been stopped without a license on-hand but able to recite their number, resulting in no trouble at all.