I know this idea was declined 4 years ago, but I'd like to try it again.
1. If you use WHMCS to bill for a variety of services beyond just hosting you clients might demand the ability to use different cards and payment options/ product or service. For example, they might want to charge hosting services to one card. But one off items would get charged on another.
2. A credit card on file might be maxed out one month and a client would prefer to add another card, without necessarily ditching the details of the card they want to use.
3. It's also just plain convenient to be able to store a few cards on file and chose when and how to use them.
The way it stands now, each time you want to use a new card, you have to lose the details of the card on file. It just creates a bit more work for the client.
Multiple Credit Card Support
Multiple Credit cards in the system and be able to let the customer choose between them to pay and selecting a default, and of course have different billing information on each cc. Also allowing us to either still enable them to delete or not delete them, and maybe even a feature for us, which will let them "delete it" from their view but keep it in our view.