Credit card 15-20% APR, debit card you make money though interest. How is not having a credit card is a poor financial decision?
Not having access to immediate credit is less useful than having access to that credit. It is probably a poor financial decision to USE credit at 15%, but having a credit card with a 15% APR is better than having NO access to immediate credit at any rate.
If you currently are speding x$ per month by way of a debit card, you could spend the exact same amount each month on a credit card, and at the end of the month pay off that credit card with the money from the bank, thereby gaining the (admiditaly minimal) intrest for having that money in the bank. Many credit cards also supply extended warantees, theft protection, travel insurance and other benifits, including points/miles/credits/cash rewards. Each of these features is available from credit cards with no fees.
It should be noted however, that carring a balance at 15% will quickly swamp the small financial gains listed above. If you cannot pay off your credit cards each month, they are best avoided.