I completely agree Nevermodern - I put as much as I can on my card too. Debit, that is. I have a credit card but only use it if I have to.
Brian's point is confused. It's quite possible that 'minumum amounts' just encourage people to spend more. You know those shops that require you to spend x amount before you can pay by card? I end up buying things I don't want just so I can pay by card.