I'm guessing the tradeoff is the thickness of the computer. They want to have this triangular shape that just barely fits the one USB-C port in the back and then tapers to a point in the front. I'm guessing that a second USB-C port in series with the first one would force them to make the back of the triangle thicker. Alternatively, placing a second USB-C port instead of the headphone jack would have worked, for the physical packaging, but then what, you need a dongle just to use a pair of headphones?

From that perspective, Apple's decisions makes some sense. What surprises me is just how lame their dongles are. $79 gets you a breakout box with another USB-C, a traditional USB, and your choice of VGA or HDMI. Really? Weak. I'd want something more like the various Thunderbolt docks that are now available (e.g., this Elgato).