> The collected data makes stock purchasing more efficient.

Why does the data need to be personally identifiable for this? Is there a reason a store can't look at total sales volume for a particular product & figure out how many more units to order?

Reasoning for storing any data outside of a GUID is so you can recover your account or key it in manually in lieu of swiping the card. Phone numbers are associated with the card here. You can punch in your number to the payment pad and leave your membership cards at home/throw them away.

Total sales volume doesn't get the same accuracy of individual shopper habits. I'm not in this sector so somebody else can correct me. Knowing what products each customer tends to buy during cold/flu season or a holiday or some opaque reason allows them to predict demand rather than react to overall trends and current stock amounts. If their data model knows that a small group of heavy purchasers of an item haven't purchased the item recently and are expected to visit the store soon, that demand can be anticipated. If a customer moves, then purchasing habits can be automatically planned for at the new store they begin shopping at rather than waiting for their sometimes infrequent purchasing habits to affect sales at the new and old locations.

A false dichotomy. It's not about today but what they can do with the data in 5+ years from now (and yes, it'll be stored for that long).

We can't know how data will be manipulated as future technology is developed.