No one at YC thinks that ideas are useless. Perhaps you're thinking of this [PG, 2005]: Actually, startup ideas are not million dollar ideas, and here's an exp

No one at YC thinks that ideas are useless. Perhaps you're thinking of this [PG, 2005]: Actually, startup ideas are not million dollar ideas, and here's an experiment you can try to prove it: just try to sell one. Nothing evolves faster than markets. The fact that there's no market for startup ideas suggests there's no demand. Which means, in the narrow sense of the word, that startup ideas are worthless. This is specific to startup business ideas, not scientific ideas. And worthless != useless. Worthless (in the narrow sense) means you can't sell it for a lot of money.