The evo hood vent is to shed off the heat from the radiator and turbocharger in the nose vicinty, ever wondered how hot that area can become, try it on an EVO, plug the hood vent with something and go for a high speed run, stop, open hood and admire some melting or burning.
Yes rain water will get into the engine bay - but in very very little amounts unless you are driving the car in monsoon season at 1 km/hr