Hotel keys

I just learned something I didn't know from Andrew Tobias' site.

When you check out of a hotel that uses cards for keys (and they all seem to do that now), do not turn the "keys" in. Take them with you and destroy them. Those little cards have on them all of the information you gave the hotel, including address and credit card numbers and expiration dates. Someone with a card reader, or employee of the hotel, can access all that information with no problem whatsoever.

There are some other useful tips there for avoiding identity theft.

Frankly, I'm still scratching my head about the hotel key thing. What useful purpose is there to putting your information on the key card to begin with? The only thing you ever use it for is to get into your room. It makes no sense.