"In the eyes of many consumers the pricing issues surrounding supermarket "loyalty" card programs can be summed up in one simple concept: those who don't have a card pay more at the register. The stores portray it in a similar manner, but call it "rewarding loyal customers" with lower prices. But few things in life are truly simple, and supermarket cards are no different."

This website offers a really good analysis of SuperMarket cards. Unless you happen to be a "good" consumer ("good" consumers nominally being a minority) you will loose something when you give up your privacy. You DO have something to hide.

Unfortunately, the guy then trys to frame the whole thing as a class warfare issue, so I can't give the site a full endorsement. Grocery stores in different neighborhoods with different income levels already stock different products at different prices, and usually the prices are cheaper in lower income neighborhoods where consumers are more price concious. This issue is far more complex then that. At issue here is WITHIN a store; WITHIN a particular socio-economic group, who are the "good" customers and who are not.

CASPIAN Consumers Against Supermarket Privacy Invasion And Numbering