Ohh, now I get it. Yeah, It does seem a waste for Zoro. Though not to Robin much. Discovering and finding out ancient origins is her specialty aboard the Merry after all. And Nami is usually the brains of it all. But since it was centered around Luffy, Sanji, and Usopp getting to the sacrificial altar, they had no choice but to give the discovering part to those three. So yeah, I agree that at least Zoro's role felt wasted.