As bookblogger has often mentioned, the flipside is that Wellcome and Octopus can now track your shopping habits. Whilst I feel a bit uneasy about that in principle, since I almost always use a credit card with my name on it to pay (so that, for example, I got HK$825 of coupons today for using my Citibank card to buy stuff at Wellcome that I was going to buy anyway) they could do that anyway, so it's no big deal.
Basically if you're worried about this stuff you pay cash. If you're not then there's about 10-12.5% to be had off your Wellcome bill if you can do it in chunks of $500+ at a time. (More if you buy wine when they have an additional 15% off for spending over $300 - but be careful: some wines have their prices ramped up during these promos).