I’ve just purchased a design using store credit, and the system required me to enter a billing phone number, address, and credit card.
I’d really rather not give up my CC details if I’m not using the CC. Nothing personal, but if I don’t give it to you, you can’t lose it. (Are you storing the CC info? I can’t tell.) Furthermore, dropping my CC info all over the internet can make it difficult to keep purchases under control.
Also, this system makes it difficult for those who don’t have a CC (due to age, nationality or other reasons) to use the system. These concerns aren’t as rare as you might think.