David of Broadway wrote:

Seems like that practice opens the door to a good deal of fare evasion.
(Person A buys an Oyster-based Travelcard and gives or sells the paper
portion to person B. Now A and B can travel on the same card at the
same time!)

Except they can't, because the paper portion has NOT VALID FOR TRAVEL
printed on it, and wouldn't operate any ticket gates.

And if you've got an Annual Travelcard you need to keep the the Gold
Record Card to get your 34% discount on leisure travel.

Cheers,

Barry