Nothing personal ZBANG, oil patch has everything under the sun to -including portable generators, attached to 8' ground rods doing nothing but wasting money. I contend that they have it done for lack of understanding grounding vs bonding. I've tossed 250.4(A) <all of it>,at those in charge, and it's easier to do it, than fight the fight.
Checking definitions was interesting -
Portable (per the 2005 handbook) describes equipment that is easily carried by personnel from one location to another.
Mobile (per the 2005 handbook) describes equipment such as vehicle-mounted generators, that is capable of being moved on wheels or rollers. The frame of a portable generator is not required to be connected to earth (ground rod, water pipe, etc.) if the generator has receptacles mounted on the generator panel and the receptacles have equipment grounding terminals bonded to the generator frame.
I know, Handbook is not code, (FPN's) just as explanatory material is not. Believe that the definitions are acurate in their proper perspective though.