Kinda along the lines of what I was thinking....still letting the air though, but keeping the wood chips out! It's gotta free it up quite a bit with the holes in the cover like that. I don't see how air really gets too easily otherwise.
If you were to look inside the air filter cover on a non vented cover you will see an area just ahead of where the air filter sits that looks like combs. That allows the air from behind the cylinder to get to the air filter. The 3700 vented cover does not have that comb area open. All the air comes from the vents in the rear.
It might be interesting to see if the 3400 air filter covers with the holes cut or drilled in the rear of the cover are sucking air from the drilled holes or if air is exiting from them. The air from the rear would be cooler for sure.