Gary, AGAIN DARN GOOD QUESTIONS
You ask "What was the advantage to this type of floating distribution system?"
Okay, these were sort of before my time, (I always speecified a Grounded system in all my years) but I have been "told" one advanatge in the early days of manufacturing was they didnt want a plant or production line to shut down if a ground fault occurred so with a "floating" non grounded system the machines and assembly line kept in production with a single ground fault. Also in a pure floating system theres "theoretically" zero voltage with respect to mother earth and zero voltage with respect to the machines or metal or conduit etc. in the vicinity. The Voltage is ONLY with respect to line to line NOT any line to earth or other metal or machines.. HOWEVER due to mutual inductive coupling and capacitance the farther away from the source and more wire runs etc the more voltage there can be developed albeit low energy.
NOW NO WARRANTY ON THIS maybe some old timer electricians and plant managers can expand on this but the above is what I was told and also is supported by the theory at least behind a floating system. In a floating system they can use detectors to sense if a leg is shorted that way they can fix it before a second leg shorts also YIKES Maybe some plant men and sparkies can comment on the advantages of the floating Delta also BECAUSE I NEVER USED THEM.....
and just arent familair with all their advantages. My Deltas had one transformer center tapped which I grounded and when I did design a 3 wire delta I still grounded one corner.