Your reciept from ther shop is your guarentee, and if you have a problem your dispute will be with the shop.
These cards are basically a marketing device, an attempt to gather names and addresses and usually get you to answer a few other questions.
They are couched in Guarentee/Warranty type phrases to scare people into sending them back.
I tried buying something in Dixons a few years back and they wanted my name and address (for the warranty). I told them I had the recipt so they didn't need my address, if I had a problem the recipt would cover me.
The first Dixons insisted on the Name and Address, so I left, the second Dixons agreed to let me buy the item without giving my name and address.
-Rd