A simple way of maybe switching on and off if the generator or charger are working.

Would be great for making accumulator systems with this, allowing to only use power when it's needed. Slightly harder and more expensive but would allow a much larger and more compact designs.

Or at least a way to check if a generator has a battery inside.

Generator-type entities can't have a circuit interface.

Power switches/inserters are not enough for your use-case? They are both circuit-controllable.

They are useful for everything that does not involve a global power connection (space platforms)