[mathw]

If you look at the history of the Tesco Clubcard (one of the early ones to appear in the UK), the data they mined from their new ability to tie purchasing trends to customers more than made up for the cost of the rewards being offered to get people to use it.

There's a reason they got so dominant over here - they were simply way ahead of the game on offering what their customers actually wanted, because they were the only people who actually had a clue.

That has of course changed now.