Once you're going 35 mph or more, air passes through the core fast enough that a fan isn't needed at all. Yes, take it off for that trip to California and get better mileage (just remember to take it with you).
At idle without a shroud the fan is just pulling in air from behind and below the radiator and pushing it under the car.
From walking speed to 35 the shroud allows only air that has passed through the core to reach the fan.
Then, why don't they come with one?
1. "none" is as cheap as it can be
2. adding 2" to the fan diameter may fix the problem (but requires more power)
