Obviously gay people aren't a necessity to the survival of the human race. Most people aren't necessary to the survival of the human race, as long as there's people with enough sense to stick the long bit in the wet bit and enough of them to keep humanity out of the shallow end of the gene pool.
But given that we're nowhere near that, it sure seems a shame to miss out on people with as good a chance as anyone of being the next Einstein/Mozart/Mohammed/(insert influential person here) just because they like people with the same genitals.
There's an argument to be made that people with major negative genetic conditions are doing humanity a service by not existing, or at least not procreating and propagating that particular condition. It's not one I would make, but it's there and it has some validity. But there's no such argument to be made about gays. It's not that we need gays, it's that why would you get rid of them? They're just people, and killing them all benefits humanity not at all.