When I worked on the payment system for an online service, we asked credit card type as an indirect way of telling the user which card types we accepted. Simply listing the card types / logos didn't turn out to prevent people from entering numbers of unsupported cards. Forcing users to select a card type cut the rate of unsupported-card-type numbers that got submitted to our system down to basically zero, and oddly also cut way down on the number of customer-service inquiries we got about whether we planned to support card type X.