At first glance, it may not seem like a worthy unique technology since the Eagle Warrior is the scout unit for Native American civilizations, but it is actually well worth researching because of the following reasons:
The Mayans have no access to cavalry, and this significantly reduces their front-line infantry's ability to survive, putting their archers at risk.
Eagle Warriors, as already implied, are the Mayan equivalent of cavalry, and thus must to some extent fulfill the roles of Light Cavalry, Knights, and even Camel Riders (they have a minor attack bonus against cavalry).
The vast majority of infantry units have an attack bonus against Eagle Warriors, and can defeat them fairly easily if out alone. Also, there is little point in using units that last only as long as it takes to create them, even if they are good at specific things.
Overall, Eagle Warriors are one of the most important Mayan units, but can easily fall prey to certain counter units. El Dorado attempts to rectify this problem and succeeds to a reasonable degree. It also helps a player not fall into the trap of relying on archers too much.
War is often the most cruel and terrifying when markedly different cultures collide. The native populations of South and Central America were shaken by the Conquistadors wearing armor, wielding steel weapons, employing war dogs, and using firearms. The blood sacrifices, rituals, and fanaticism of the Americans unsettled the Spanish, in turn. The Mayans proved very tough opponents when whipped into battle frenzy in defense of their hidden wealth.