There's also an issue of anonymity and trust. There are plenty of websites from which I'd like to make purchases (some of an unflattering disposition) that may continue to bill after cancellation requests (because they know nobody would take them to court for fear of having their names associated) or that I just don't want bound to my name.

How many times have you found yourself shying away from a site because you were worried about what they'd do with your credit card number? This isn't a perfect solution (especially with the limited pool of numbers), but it's nice.