Check out 250.32(B). It gives you two options: with or without an equip. ground.
Option (2) is without the equipment grounding conductor, and the rules for this option include: "there are no continuous metallic paths bonded to the grounding system in each building or structure involved".
Sounds like you are right.
The reasoning isn't so much for the eventuality of an open neutral, but more for the fact of a definite parallel path with the grounded conductor, which is to be avioded. How would you like to disconnect the shield on your cable TV, and pull an arc? Or, work on the plumbing, and get rapped?
