Bitcoin can be useful even if a store point-of-sale system is never implemented.

But to go with your scenario, I see two options:

1: The clerk scans his items, he goes to the payment device (the thing people swipe credit cards with now). The screen gives options for the buttons indicating types of transactions, one of them says "Bitcoin". Adam presses the button by bitcoins. The screen brings up a QR code bitcoin amount, store name, and address (all in a bitcoin standardized format). Adam pulls out his smartphone, opens the bitcoin app, zaps the code, presses the confirm button on his phone. After a second the screen says "transaction received" or something like that, Adam takes his stuff and walks out. (The store name is not nescesary for the bitcoin transaction, but it makes record keeping nicer so when you look back at your wallet you know what that transaction was from.)

2: The clerk scans his items, Adam swipes a card just like he would a credit card. This is linked to an account with bitcoins, but the store clerk doesn't need to know about all that, since everything happens in the backend to do the conversion and the store is electronically sent dollars, just like a credit card.

Why I say bitcoin can be useful without this setup is that bitcoins can function as a savings tool, like gold does today. People who want to save money convert it to bitcoin on an exchange or with somebody local in person, and when they are done saving they convert it back on the exchange. Bitcoin has the advantage over gold in that it is harder to fake and it is easier to trade with people farther away.

Bitcoin is also very nice for internet purchases. And for doing long-distance work (like programming or digital artwork) where there is no reason for the employer and employee to be restricted to dealing only with people in their own country.