National Australia Bank faces an uphill battle persuading retail customers that its revised credit card rewards programs still offer value to cardholders.A study of NAB credit cards linked to rewards programs by financial products research site Mozo found that points earn rates are set to plummet by as much as 47 per cent when changes take effect on 13 November.It's a tumultuous time for NAB's credit card business, which has also announced the ditching of its distribution deal with American Express.NAB will cease marketing AMEX companion cards on 13 November and the entire portfolio is expected to be phased out by the end of February next year."The changes announced by NAB are a massive blow to AMEX cardholders and their rewards earning capacity," said Mozo director, Kirsty Lamont."NAB cardholders saw points earn rates plummet in May of this year and the bad news only continues with the abolition of American Express and diminished rewards earning capacity," Lamont said.NAB and the other major banks have been restructuring their card businesses this year after the RBA's Payments System Board constrained their ability to collect interchange fees through credit card transactions.Most have responded by crunching earn rates on rewards programs and rationalising the range of cards they offer.Other issuers of AMEX cards such as Westpac and CBA slashed the earn rates offered to cardholders in May.The Mozo study found that the value of rewards programs attached to credit cards across the industry has fallen by 63 per cent in the past 12 months, with the average cardholder now spending A$19,000 to earn only $27 in rewards.Grant Halverson, a former CEO of Diners' Club in Australia, said rewards schemes were almost dead-in-the-water as marketing devices because they offered little or no value to cardholders."In the mid-1990s, it was common for cardholders to earn a return flight between Melbourne and Sydney after spending $4000 on their cards," he said."Today, a cardholder needs to spend on some cards at least $25,000 to get the same reward." For more than two decades NAB has consistently failed to match the growth of other major banks in credit cards.Efforts to revive profitability of the business since 2011 were undermined by regulatory changes to interchange fees and the rapid decline in the number of cardholders revolving balances between statement periods.While rewards programs were traditionally used to boost revolve rates, their diminishing value to cardholders has further eroded interest revenue on the cards portfolio.