For me, the credit cards wouldn't be the most useful, but rather the membership cards like Costco, Zipcar, gym, etc. The problem is that none of these cards are

For me, the credit cards wouldn't be the most useful, but rather the membership cards like Costco, Zipcar, gym, etc. The problem is that none of these cards are standardized (Costco has a photo of the member, Zipcar is a prox card, gym card is scanned by barcode rather than magnetically, etc.)