Cabin Air Filters and Convertibles

[Giller]

Hey guys....regularly change my cabin air filter on my other cars - but does one need to do that on a convertible? In a closed car, the air filter can make a big difference, but on an open topped cruiser - what exactly does the cabin air filter do? Does it need changed regularly like a closed vehicle would?
Sorry, but having trouble visualizing the benefit as all the dirt and smells and bugs etc just come in through the big, non-filtered hole where the roof normally is. What am I missing.
Thanks!