Yes. As soon as you use an untrusted computer, there is no telling what you are vulnerable to. But: (1) people frequently (and rightly) do that anyway, so we

Yes. As soon as you use an untrusted computer, there is no telling what you are vulnerable to. But: (1) people frequently (and rightly) do that anyway, so we might as well make it less bad. (2) Building a trustworthy computer is largely about isolating different components in a way that hurts malware more than it does the user-experience. Sticking keys on a USB key is one instance of such isolation.