If for the shimmer look, maybe you right, but I never notice any irritate those light bring to my eyes. Another benefit of those diffuser is prevent kids not looking direct into the lights.I have them and dont really share that opinion. Regarding coral growth i cant say much. But the shimmer they provide makes them worth it in my eyes
If for the look, I LOVE Metal Halide! Those light make the tank look crisp with LED can't never have.