For exaple something that happens almost every day in the business world: Somebody walks up to you and gives you a USB flash drive with some files. And please, don't say "Airdrop" now, since it is a proprietary Apple protocol that is not supported by the majority of notebooks in the real world.
I still haven't heard a single good reason why Apple shouldn't build an additional USB port into the thing.
Flash Drives with USB-C will be here.....
And that's more a problem with the type of USB port, not that there's only 1.
If the person has an iOS/OSX device, why not ask them to AirDrop it?
There are also numerous other ways to wirelessly transfer a file via bluetooth and/or wifi.
But again, this is a problem with the USB-C port, not the lack of a second. I would guess it's not going to take long for USB-C thumb drives to pop up.
As for not adding a second, I've already said this: perhaps they just can't fit another in and still get the battery life they want. My guess is, those internals are packed tightly and every inch is taken up.
Or maybe I'm wrong and you'll get a second port in the future. Still doesn't help your scenario if it's USB-C.