Right, for the various caveats "what is ground, anyway?", as has been covered so far (i.e., at AC, for various styles of termination).
For example, the line itself may have capacitance to surroundings, or may be required to be shielded/grounded to facility or apparatus ground, and therefore pick up CM voltage with respect to one or both transceivers. In that case, double isolation is preferred (or required even). The same goes for the isolation barrier itself, which has to have some capacitance (no isolation is ideal).
Tim