It is... On a normal desktop. For laptops, however, there isn't really an option, as nVidia and AMD chipsets rarely come as a USB external device.
What do you mean? I can disable my laptop GPU and enable it as I please in device manager...Device Manager doesn't just deal with USB sticks....
You are failing to grasp the situation of this post, however. For the most part, laptops only come with ONE graphics adapter. By disabling it, you cannot make use of any graphics adapter. Even you, yourself, suggested disabling the onboard graphics in order to use an nVidia card. On a laptop, you are effectively disabling the only graphics adapter available SHORT OF AN USB EXTERNAL GRAPHICS ADAPTER.
To my knowledge, there is currently no other means to provide graphics to a laptop beyond the onboard graphics adapter except with a USB to VGA/DVI Adapter. And to my knowledge, most of those are designed for simple Windows Desktop, NOT for serious GPU/Gaming.
And no, I am not thinking oboard graphics adapter != nVidia card, or AMD card. The graphics card, be it intel, nVidia or AMD is still an ONBOARD, INTEGRATED graphics adapter on a laptop. It isn't two separate things.