In my previous example (LTJ-811 20mA@100% vs 25mA@50%) the LEDs will appear to have the same brightness (relative to each other) regardless of the ambient light. In the dark they look the same, and in bright light they look the same.

I accept that in direct sunlight neither of them might be bright enough to be useful in a sign, but I couldn't push the IF any further in the example because the info in the datasheet doesn't extend beyond IF=25mA, and I couldn't run the numbers without info on the (non-)linearity.

And I also accept that if you have to push the IF out beyond its linear region (to get maximum intensity) then reducing the duty cycle may not be viable because of the non-linearity.

What I am adamant about, however, is that multiplexing/reduced duty cycle is useful for more than just "saving ICs".