One other thing that I just thought of:

[Dishonest mode on]

If you are a business owner, and put through a large amount of charges every month, you could easily put another 3 or 4 thousand on the card and "forget" to list is as income at then end of the year. One of my customers has on average $175,000 to $200,000 on his business card every month (him and 10 or so users) it's his business so it would be a simple matter for him to take almost another $60,000 out of the business a year and not pay taxes on it.

[Dishonest mode off]

The above would have very limited application in the world but I could see it being useful for some people.

-Dave