Bruce Schneier on Authentication and Expiration when purchasing things online. “The e-commerce site wants me to live in their database forever. They want to market to me, and entice me to come back. They want to sell my information.”
There are a few different methods for managing this. One is to pick a couple of sites, and buy everything from them. This, of course, can end up being a costlier route, as it precludes comparison shopping. Personally, this is one of the reasons I prefer using PayPal for online purchases because it abstracts my sensitive data from the vendor. I can buy things from a hundred vendors using Paypal, and there is still only one copy of my credit card number stored.