Stores more and more frequently offer better prices to customers who obtain one of their "membership" cards. Obviously such cards can be used to identify the shopper, collect data based on their purchase habits, and obviously be of great use to the company in determining prices, sales, and inventory. Even if you don't associate your real identity with the card, the company can still use the data to know what items are purchased together and with what frequency, etc.
The question is, do these stores purposely use such data to go beyond simple market research and instead use the information in ways that are detrimental to the customer?
I am skeptical of the need for these cards because most of the data they need can be simply obtained from their own cash register. The only information you gather with the card that you cannot without, is the association of separate purchasing events. In other words, if I pay cash for Armadillo Food in one shopping trip, and cash for canned tuna in another, the store has no real way to know that those two items were obtained by the same person.
So why do more companies feel that such cards are needed? I don't believe that the discounts are real, but rather prices are inflated for customers without cards, thus making the card a benefit only in allowing the customer access to the normal price.