George Little,
I would question if it is required of not because it does not make any sense not to.
If you draw a simple diagram of an ungrounded delta feeding 2 3p breaker2 that is supply power to motors in different part of a facility 100 feet apart. The motor winding in one of the motors fails and the 'A' phased contacts the frame of the motor which is grounded mechanically. Will there be a ground fault? No, because you have not provided a path for current to flow back to the source. Do you know that it has failed? Most likely not because what indications would you have?
Now, the other motor just happens to have a failing winding also except that it involves the 'B' phase. Now the 'A' and 'B' phase have contact with the ground through the metal frames of each motor. Does the current stop there? No. The current is going to seek a conductive path through the facility form one motor to the other, though anything that is conductive.
You never want that to happen so don't even question is GF indicating lights of any other approved system is optional. It doesn’t make sense not to.
BUT, GF indicating lights are useless unless there is a qualified person present to react to a GF indication. That's when he springs into action to identify and isolated the the grounded line be there is a catastrophic failure.
So, it doesn’t end with the lights but must follow through with personal who are qualified to recognize and respond to the event.
Dave
