With mortgage, that’s not unreasonable. After all, you will always be bearing some risk as your APR will never be below the risk-free rate. But for credit cards, if you just store the money in the risk-free rate (3 month T-notes), you will be strictly making more money without bearing any extra risk. In affect, it’s a free lunch: it is strictly superior. You could probably even program such a system and never have to think about it. Here’s an example using 1 credit card that charges interest after a month:

1. Initially, buy 3 month T-notes equal to your max credit limit.

2. At the end of the month, transfer the dollar amount of your current credit balance and pay off your card.

3. Top off your T-bills to maintain the max credit limit amount.

4. Profit!

You just scammed the credit card companies out of a month of interest! The strategy looks even better with cash back or airline miles or points or whatever.