HID Light Diffusers, What's Their Purpose & Do They Work?

Ive been curious about these light diffusers, what is your opinion on them? Do they really work? I'll be doing SOG (keeping the plants small between 20-30") in a pretty small grow box, only 6 1/2'x4'x2', I'll be using a HTG 6" Euro Reflector air cooled hood with two bulbs end to end, a 400W CMH + 250W HPS, with this setup would I even benefit from using light diffusers?

They reduce the lumen output of your light. Seems like a really horrible gimmick to me. Useful for pictures, not so much for growing a plant.