I wonder though, if you had a modern vehicle with an computer controlled fuel injected efficient 4 cylinder engine and compared to an inexpensive carburated generator, which one would burn fewer gallons per hour.
I think parasitic load would come into play as well.
In any event, the main benefits in my opinion are having only one engine to maintain, having it with you wherever you have your car, and larger fuel capacity than most standalone generators. The big disadvantage is, of course, you can have power or transportation, but not both at the same time.