I suppose the point being made is by grounding a portable generator you are creating a fault path for no particularly good reason. Let's look again at why we ground electrical systems (250.4(A)(1))
(1) Electrical System Grounding.Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation
You are not going to see line surges or contact with higher voltage lines if you are using a generator and if your generator gets hit by lightning you are toast anyway.
