I can see one use for the engine generator, and that is when on the road, all electrical appliances are usable, so the roof A/C can be used, or maybe one can pull over and run the microwave. Even then, it isn't a bad idea to run the onboard generator just for upkeep reasons.
Time will tell about how well this works. Because of the new EPA regs, the new engineering around DEF and DPF is relatively new. Maybe if there were a way to trigger a DPF regen cycle manually when one knows they will be at highway speeds for a bit, it might help things.
If given the option, I'd take the engine generator even if the rig had another genset onboard, just for the flexibility.
