I actually did this exact mod myself last month. It's not blocking airflow; as Scott mentioned, it's allowing the single rad fan to pull air across the entire rad. Contrary to popular belief, even at speed, air is reluctant to go through your rad unless your fan is running. Rad fans pull air away from the rad creating a low-pressure zone; this is what encourages the air in front of your rad to come through the fins. Done properly, that shroud would actually improve airflow.

Doing it properly would involve sealing it up around the edges though.