Really? I thought all you had to do was scratch the surface. If that is the case then all used ones are cut...correct?
I don't have any first hand experience redeeming these cards but I would think you can redeem it without cutting the card in half. Everyone has their own way of redeeming and I recall scenarios where people were selling peeled/redeemed items and they didn't put the peeled hologram back on (extra value for resale).
An argument could probably be made about privacy for the user not putting the redeemed holo/key back on the item, but it definitely has more value if redeemed and is as intact as possible.