I've actually had the opposite experience as you. Having my debit card keeps an easy running tally which is difficult to do unless you only leave with small sums of cash in your pockets. However, when you only have a small amount of cash you run into the "just short" effect, where all you need is a few dollars more to get what you want and you can afford it but can't get it. To me, the potential opportunity cost is just not worth it.