Today, if I use public computer, then common scenario is it takes three steps. 1-log in, 2-do what I want, 3-log out or close the browser. When using cards, two more steps will be added. I need to import my card before log in, and I need to erase my card after log out. The more clean-up needed, the more possibility you forget something and someone to misuse your account. Moreover, leaving your card not erased on a public computer poses a real trouble, because many people who uses the same computer after you did is capable of using your identity. You wont be aware until you notice some changes in your bank account balance, will you?