Not two days ago I presented at a conference where I had to plug an HDMI cable into my laptop, and in the middle of setting that up, I transferred over a backup copy of my presentation to the house laptop on a USB stick.
Now maybe I am not a professional in your eyes. But I was wearing a suit!
I'm fairly certain that a USB-c to standard USB port will also be a dongle that every Macbook Pro owner will need to carry. Apple should bundle 2 with every Macbook Pro, but the whiners would still complain...
Yes, you need either a new cable, a new flash drive, or an adapter. But don't pretend that makes this laptop totally unsuitable for your use.
The common themes to wanting a Macbook Pro is that you want more grunt and more I/O, and you have the money to pay for it. A common need among this group is exchanging large data sets, and not being willing to use iCloud/Dropbox/Google Drive to do it for a variety of reasons, like speed, convenience, confidentiality. These are also people who often want to do this in a variety of locations.
It is also a massive pain when someone wants to give you something on a usb stick. Same goes for people in publishing and other content creation industries. People who want to plug an external monitor in to give a ppt without needing a special dongle.