Think of it this way....... If you have a jar with a light bulb in it, and half of the jar reflects light to the other side of the jar which is clear, you get 100% of the light on the clear side of the jar.
If you have a totally clear jar, then the light gets spread out so half is on one side and half on the other. Unless you grow from the ceiling down as well, the plants will lose out on a lot of light!
Mostly reflectors are used unless you are growing gigantic plants (We're talking 4 feet tall plus) and you can then hang them vertically to the sides of the plants instead of above them. Light above a plant, not to the side of them, need a reflector.