My card has my picture. If an employee is going to look at it, then no reader needed. And you point out that they used to do this. What is the reader accomplishing then?

You are creating a scenario that does not even exist. The card readers are already implemented in stores, they have tested the time savings. The card reader process is essentially the same. You flash your barcode to the reader instead of the person. Why would there be a line, its not like you are crossing the Berlin wall, its just a check a more rigid check at entrance instead of checkout.

Of course your picture is on your card but I think we would all agree that a picture on a 3x2 credit card that can be B&W is inferior to the color picture displayed on an ipad. Having an associate look at a picture on the back of a credit card is slow.

It depends on store location but they have always had varying degrees of checking that you are the owner of the card. Costco makes a majority of their profit from Membership fees, hence the desire to enforce the long standing rule.