He says why. With swiping, you can often do the swipe while the clerk is ringing up your groceries and put your wallet away as opposed to having to do the transaction at the end (as you also need to do if you use your phone).
He's not wrong although this is pretty far down my list of life's annoyances :-)
The swipe transaction system doesn't provide a mechanism to authorize a transaction for a specific amount, unlike the chip system. That is - you swipe, and the merchant uses the details on the magstripe to make some arbitrary transaction later on - potentially weeks or months later as I've found out in the very few places in the UK that still run offline transaction systems. With a chip system, you authorize a specific amount and no more within a dedicated machine, and the merchant doesn't get a hold of all the details necessary to make another transaction.
I imagine they'll void or otherwise adjust the transaction. I've never had an issue. Frankly I'm more likely to notice something while I'm leaving the store if something didn't seem quite right and I start looking over the receipt.
He's not wrong although this is pretty far down my list of life's annoyances :-)