When a bank creates a credit card account the revolving line of credit is typically bundled with others into a security. Citi earns fees when selling these securities. The line of credit for a specific card account remains in the same loan pool for the duration of the security. Different from mortage backed securities the issuer retains ownership of the credit card account because revolving credit lines do not have a predetermined amortization rate or payoff date. The issuer distributes interest payments to the owners of the securities, and uses principal repayments to fund loans to cardholders who draw more of their credit line during the life of the pool.

In this context you can see that a card which is used briefly and then paid in full is useful because it generates early cashflow for the loan pool with virtually no risk of default. The aquisition cost of the new accounts, including the miles, points or affiliate payments are no doubt built into the pricing of the securities.

It would be better for Citi if you kept and used the card long term, because Citi would earn fees (merchant transaction fees, annual fees, foreign transaction fees, various penalty fees). It would only be better for the security holders if you carried a balance, because they only receive interest, not fees.

You can read more about this in the FDIC Credit Card Securitization Manual

http://www.fdic.gov/regulations/exam...ecuritization/

...and you can read ample opinions about this process, including the "off balance sheet" variants using search term: credit card securitization