I disagree. Whether the air entering the cabin makes a single pass in normal operation or multiple passes when in recirculation mode, it's still being filtered. The degree of that filtration depends on the quality of the filter. In theory, a vehicle with a HEPA filter sitting still with the HVAC system on recirculation mode could achieve near-perfect air quality. Obviously, that's not possible in the real world, but the idea that the filter is there just to protect the HVAC system is flawed. Why do you think so many cabin filters are sold with activated charcoal? Do HVAC systems have a sense of smell? The filter is there to improve air quality in the cabin for the passengers, the fact that the blower is also located in the cabin means it also benefits from the filtration, but the system is primarily designed for passenger comfort.