We have 3 Maxxair II vent covers installed on our rig with vent fans on two of them. IMO, they should come standard from the factory since they really don't cost all that much for the many benefits they provide.
We do close them when running the AC unit, but they are pretty much open all the time with at least one vent fan running as well. Been through some major downpours and never a problem with rain getting in our rig. Have experienced a few extended dust storms and they did allow some of the dust to penetrate our rig's interior, but vent covers aren't designed to prevent that anyway.