Remember, what it LOOKS like it does and what it ACTUALLY does is often two different things. The study of aerodynamics is a complicated science, and just looking at what we think will be airflow is pointless.
Even if it did increase airflow and MPG's with a trailer attached, which is doubtful, also consider the extra weight of the contraption will decrease MPG's as will driving around with the thing without the trailer.
Also consider the air dams you see on tractor trailers have been scientifically tested, and even a minuscule improvement will pay off on a vehicle that is driven 100,000 + miles yearly for 8 to 10 years.
