Caspian

Consumers Against Supermarket Privacy Invasion and Numbering.
This starts out sounding like a flaky organization with a bit too much conspiracy theory on their minds. What they come up with is pretty surprising. Among other things, Customer loyalty card programs are very expensive to operate. That cost is passed on to us as consumers.

"For many consumers, the pricing issues surrounding supermarket "loyalty" card programs can be summed up in one simple concept: those without a card pay more at the register. The stores portray it the same way, but call it "rewarding loyal customers" with lower prices. Though this sounds simple, few things in life are as simple as they seem and supermarket cards are no different. "

http://www.nocards.org/overview/index.shtml

Here is a follow-up. This is a pricing study comparing "card" stores with loyalty programs to traditional "non-card" stores. The results are pretty amazing. They are also similar to a very unscientific comparison done in my own home. Admittedly, these are unscientific studies that compare on only one factor. My daughter (currently studying AP Statistics) would argue that single factor studies are limited in their usefulness. Still, there is something to think about here. This doesn't even bring into account the privacy concerns raised by using the cards. The industry is experimenting with RFID cards which would further expose you to privacy concerns.

http://www.nocards.org/savings/regular_price_study.shtml