Upload & Sell: On
There isn't any use for those cards. Canon doesn't ask for them for service. Think of it just as a serial number card that accompanies the lens through the manufacturing process. The only thing you need for service is the receipt.
Technically, the warranty is non-transferable. If you buy a used lens and the previous owner's name is on the receipt, they could simply tell you that the warranty is not valid. In practice, canon rarely makes an issue of it so long as the receipt falls within the valid date range. Besides, if they question you, you could always say that it was purchased as a gift.
But yeah.... the blank warranty cards mean nothing. Those that boast about having a blank warranty card when selling the lens are either misinformed about how the system actually works, or they are intentionally trying to mislead potential buyers.