This would be acceptable if it meant adding one more shader, but with "modern" graphics APIs forcing us to sometimes have thousands of permutations for the same shader, every variant you add multiplies that count by 2x.

We also don't have an infinite amount of time to work on each shader. You profile on the hardware you care about, and if the choice you've made is slower on some imaginary future processor, so be it - hopefully that processor is faster enough that this doesn't matter.