The card is not the shopper
This (DISPATCHES FROM GROUND ZERO - PRICE CHECK ON PRIVACY, By Joe Salkowski) is why you don't want a card associated with your purchases.
Officials from Smith's and Safeway say their stores don't share club card data with would-be advertisers. So if Nabisco wants to send the Keebler Elves out knocking on the doors of Wheat Thin eaters, they'd have to hire Smith's or Safeway to run the campaign instead of buying the lists and doing it themselves.
That's nice to hear. But it doesn't mean the stores won't misuse your information. Just ask Robert Rivera, a Los Angeles resident who says his local grocery store threatened to use his purchase records against him in court (emphasis mine).
Joe Salkowski details Rivera's lawsuit, and goes on about several other scary scenarios - actual and possible.
And for good measure, many more reasons to shred that card in No Cards Shoppers.