I can't speak for the first poster, but I do know that for me personally, I prefer to use a debit card for purchases I would have made with cash and not a credit card. I fully understand that should I pay off the credit card in full each month it's the same thing, I have never had the discipline to do so, the balance keeps creeping up. With a debit card there isn't any chance of that. With a debit card if the money isn't there I don't make the purchase, if I was using a credit card I probably would. I carry multiple cards of each type, but really prefer using the debit card (fully understanding I don't have the same protection of a credit card, etc.)

Another reason could be for people who don't want any additional credit cards, or can't easily qualify for a mileage earning one.