Hmmm...you think American Airlines going over at 33,000 will be able to see me? Quick! A plastic toy 3 miles below us. Do something. Obviously it would be for the ones lower down, but even then at 5,000' there's not much of a factor. It would have to be more for quads to avoid each other.
I'm not quite sure what we need a 3SM light for when we can't even fly above 400' day or night. Still not supposed to be in their airspace. It would have to be red on the left and green on the right, but there's hardly any distance as far as wingspan on the quads. Helicopter is just a red strobe on the tail. It sounds good on paper, but I'm not sure about the practicality here.
In the U.S.:
It's needed because the FAA says it's needed.
We can argue about the logic and the rationale, but if we want to stay legal, we need a 3 SM light.