The value of privacy
Getting privacy right is tricky. People say that they want lots of privacy, but their behavior often tells us that they really don't value their privacy that much. If you promise to email someone a weekly cartoon, for example, they'll often give you lots of personal information that they claim they want to keep private.
The club cards that grocery stores are another example of this. The stores essentially pay you to let them track your purchases; they just pay you in discounts instead of cash. I was fairly surprised recently when I learned exactly how much stores pay you to let them track your purchases.
Voltage has social event every Friday afternoon. Someone buys a reasonable amount of food and drink for this event and they get reimbursed by Voltage. I bought the supplies for one of these events a week or so ago and was somewhat surprised to see that the total went from about $110 to about $85 with the discounts that my wife's club card gave me. That's almost 20 percent of the purchase.
I don't know how representative that single data point is, but if a grocery store is willing to give you that much of a discount if you let them track what you're buying, people must be fairly unwilling to let stores do this.