If we trade credit cards for a day, you'll buy lots of stuff that you would not have otherwise bought.
Since you would not have otherwise bought it, that reveals that the stuff was not as valuable as the stuff you bought before you had my credit card.
And I will do the same. I'll buy lots of computer games with your card that I had looked at longingly last week. I didn't buy them last week, though, because I bought stuff that had more value--stuff that was necessary.
You and I will spend lots of money inefficiently.
Oh, now let's do the same with health care credit cards. Voila! National health insurance!