[SiVal]

only really useable as an auditory messaging system

What? When I was a kid, it was common for camping flashlights to have pressure buttons on the side, which was intended for (and we used for) morse code. Ships used bright, shuttered lights for Morse code, soldiers used it, scouts used it, kids communicated with friends in the neighborhood at night.

[sverige]

OK, yes, but it's still the rhythm of the flashes that are being interpreted, not _ .... .. ...

Plus, I am willing to bet that far more messages using Morse are sent over radio than with lights.