It is also the port that is used by none. I own nothing that uses USB-C, not even my own iPad, iPhone, and Macbook.
The USB-C is good, but its too early to fully implement it. Its like going HDMI only in 2001.
This is the PRO machine after all, they could go USB-C only with entry level stuff like Macbook Air.
There are plenty of people using USB-C now and it is only building up momentum as time goes by. USB-C has been available since early 2015 in a usable form. We are 3-1/2 years in at this point and although Apple might have been too early in 2015 when it introduced the 12" MacBook, it certainly is not now.
Any Lightning-equipped iOS device can use USB-C. I purchased Apple's USB-C to Lightning cable and a 30-watt charger for my iPad Pro (once you go fast charging, you will not go back) which means I can connect my iPad or iPhone to a USB-C port on a Mac or a Windows PC. It was an investment, to be sure, but well worth it.
I use a Vantec USB-C 2.5" enclosure for a spare Samsung SSD I had pulled from an old system. For $25, anyone can repurpose a 2.5" or 3.5" drive (SATA SSD or HDD) for their Time Machine or Super Duper back up. I also purchased a few vital cables to ensure that I could connect to my older USB 2.0-equipped devices - a Focusrite Scarlett 2i4 and a Zoom H1.
Device vendors are offering USB-C cables with their products now and some are moving to USB-C ports on the device itself, while they bundle in a USB-C to USB Type-A connector cable to help ease the transition for users still on USB-A.
For a Pro machine, Apple decided to offer the fastest, most versatile port available to give their Pro users the most versatility...USB Type-A is not it. Apple also debuted USB-C on an entry level computer, the 12" MacBook, which should have made it obvious what was coming down the road.