As the motor warms, the silicon inside the clutch expands, and the clutch grabs harder. So the clutch will slip less as the motor heats up. Without the shroud, much of the air around the fan, will just spin around the fan, and not be pulled through the radiator. They are there for a reason. It's similar to the way sound travels around a speaker cone that is not in an enclosure, killing its bass response.
