Actually, I often use cash to buy stuff precisely because I do care about my anonymity. Considering how apathetic the public is to the 4th Amendment, and considering how absolutely rabid the government is about finding out information, cash is the only thing left to preserve some level of dignity.
I won't call it a probability, merely a possibility, but sometime in the future, it could well happen that eating lunch at McDonalds too many times per week could adversely affect health insurance rates. Or maybe you buy some booze at a liquor store and become a target for being pulled over on the off chance you also drank the booze and drove. Or maybe you bought a particular book 5 years ago that becomes illegal and subject to retroactive prosecution.
I'm not saying I expect these things to happen, just that they are possibilities. Of course in the 90s, if you asked me if I expected the Feds to spy on all Americans, I would thought that a foil hat concept, but look at where we are today. Anyway, using cash is a method to protect against future government abuse and when you look at how the US Federal Government operates in such fundamentally un-American ways, there's certainly no harm in taking steps to protect yourself from it. Minimizing your purchase history is a good place to start. That takes cash.