Not really worthless. The Ethereal card, for example, still going for over $200 on EBay. Lot of pet collectors are wary of buying from the hackers, either because they think it's cheating or are worried Blizz might revoke their pet sometime in the future.
I recently bought a 60-day Game Card from Amazon, legitimate, box still sealed and card unscratched. When I tried to enter the code online, it got repeatedly rejected. Opened a ticket that evening with photo of my scratched card attached, the number clearly visible. Usual disclaimer about replies taking 24 to 48 hours. When I fired up WoW the next morning, already had a response saying they'd accepted the card and posted the 60 days.
Later I did some googling and found some websites that claim to generate valid Game Card numbers, or post lists of them. Guessing Blizz is going through extra validation. If you decide to use or sell your TCG cards, photos should serve as proof of ownership even if some hacker already used that number.