When reader WhataBummer posted
on Fifth Third's Duo MasterCard which can function both as a credit card and a debit card, I didn't see the usefulness. But this AP article
does show how it can be useful. It can allow those who prefer using credit cards for purchases to avoid having to have an ATM card.
To draw funds from the checking account, the cardholder would select the debit option at the register and enter a PIN. The card can also be used to make withdrawals at ATMs.
To charge a purchase on the credit card account, customers would select the credit option and sign to complete the transaction. They would no longer be able to sign for debit card purchases.
The article also does a good job at describing the differences between debit card PIN-based purchases, debit card signature-based purchases and credit card purchases.