The picture ID is what ties everything together. Without that they have no way of knowing who you are. You would think they could use a database to list state registration numbers, with a name, and the only thing you would have to carry is a photo id. Much like the tag offices do.
I don't understand why we have to be inconvenienced when the solution is as simple as this. I always carry my ID on me but to require me to have a copy of the registration just seems pointless since they know who owns it just by looking it up. The copy of the registration could be falsified anyways just set up the database and you'll know for sure. Spend some of our money your taking and do it already, and don't require it until you've done it right.