I am effectively experiencing the same problem mentioned in this thread from several years ago discussing a much older revision of the XPS 13:  http://en.community.dell.com/techcenter/os-applications/f/4613/t/19491324

The left USB port seems to power devices and function with USB 2.0 sticks but cannot mount my USB 3.0 WD My Passport Ultra drive.  It works properly on the right USB port.  Similarly, the left USB port is unable to provide  a data connection when I use the Dell J1GH5 USB 3.0 to gigabit ethernet adapter.  Again, the ethernet connection works properly on the other side.  With both non functioning devices, the port is able to power them up without a problem.  I'm forced to use the USB 3.0 ethernet adapter as I'm unable to get the USB type-C ethernet adapter that came with my laptop to function under Linux.

Edit:  Toggling the powershare settings in the BIOS seem to have at least temporarily alleviated this issue, but it's intermittent and hard for me to pin down.