Whilst I agree in general terms about privacy, a store card reveals that I eat food and use petrol and that I need to buy it regularly. I know these places like to collect data and that there's a natural superstition about the intention but I'm not sure it's info that amounts to much in the scheme of things. I suppose they can use it to send me stuff which goes straight in the bin unless it comes with a pre-paid envelope in which case I rip up the stuff and return it
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Medical/security data is another matter.