Hey, just so you know, the plastic bag doesn't actually fix demagnetized cards. What it does is helps kinda smooth over rough patches in the magnetic strip. If the magnetic strip is truly demagnetized, the plastic bag sadly won't do anything.
On the other hand, a couple other things will do this same trick. A little thermal tape from the register (most registers now use thermal tape, that's why they fade). And for all you men out there who have an issue with their cards wearing out quickly in your wallet, if you put a little scotch tape across the magnetic strip of a new card, it'll last longer. Tape will also help you stretch out a card that's already starting to go so it'll last until you get a replacement. Btw, I'm a cashier, which is why I know all these nifty tricks. =)
Back to the plastic bags, some horse trainers use it for desensitizing horses so they don't spook when one goes flying by. I know this is kind of a niche use, but if you ever get to see this for yourself, it's pretty interesting to see how scared a 1/2 ton animal is of a little piece of plastic. And for anyone who decides to toss them into recycling other than the recycling bins found at various stores, please cut the handles before recycling. Bags that make it loose onto the streets or into waterways with their handles intact are exponentially more dangerous to strays and wildlife as the handles can wrap around necks or limbs, causing strangulation or loss of circulation. Otherwise, have fun getting creative!