You are required by regulation to have it. There is, as you describe, a convenience procedure intended to help people who forget their ID. You could still be tu

You are required by regulation to have it. There is, as you describe, a convenience procedure intended to help people who forget their ID. You could still be turned away if you do this frequently or they otherwise have reason to doubt the legitimacy of it. And they don't have time for everyone in line to do it--if they did, this exception would go away completely.