Kens answer is right on, conventionally thinking, there's some thinking now that usable spectrum being only red/blue may not be the case...
It certainly stands to reason that in the billions of years that plants have relied on the sun, that they may derive something, from the unusable spectrum.
There's some interesting results coming out of the MMJ field that are showing real results from UV lights.
I think of it like nutrients, N-P-K is like red/blue, the bulk of plants needs are meet here, but there's micro and even macro nutrients, on top of that there's been recent discoveries that plants use more elements than we ever thought, on the micro/macro scale.
Who's to say what we will come to learn, know, and accept in the future.
