"Nectar particularly interesting as it is a system shared by several Big Stores."
So they all get to share in that shopping information under the pretense of giving the user a single discount card to "manage". There are plenty of stories regarding the sorts of information that can be gleaned about a person from their shopping. The discounts given, which used to be sales when the store got a good buy on something, are a drop in the bucket compared to the value of the information they pull in. It's bad enough to use one loyalty card per store, but to give a bunch of stores insight into your purchasing habits is really scary.