If there is no path back to the source, where is the danger? That is the theory of isolation transformers on lab benches.
In that situation there is no "hot" or "neutral", just 2 conductors with some volts between them. There is no reference to ground. You can be sitting on a metal bench in a puddle and there is no fault path unless you get phase to phase.
We ground services because of the things that can happen to overhead lines (lightning strikes, contact with high voltage secondaries, surges etc). You don't have that exposure with a portable generator.
