Yubikey and Google Auth, while accomplishing the same thing, are different and not mutually exclusive in the sense that each has their advantages.

First and foremost, one could argue Yubikey is a bit more secure than Google Auth.  (But for practical purposes, it's largely irrelevant).  Additionally, Google Auth is not supported on all devices.  Yubikey, also, is not supported on all devices (namely, those devices without a USB port).  Yubikey is more convenient in terms of entering your OTP, less convenient since you have to carry something around in addition to your phone (for Google Auth).

I would vote to add the ability to use one or the other, not just Google Auth.  I use Yubikey in several places and Google Auth in other places, depending on the situation and I find them both useful in their own ways, while neither of them are perfect, sadly.  Now... Google Auth would be awesome if paired with NFC and the computers I use had NFC readers.