[vectormax]

IMHO, the cabin air filter is a marketing scam. I breath the outside air unfiltered all the time. Why do I need a filter when I'm in the car?

Seems like a good idea to me. The dry air and Santa Ana winds means lots of fine dust and debris on the road, that gets kicked up by cars moving at 65mph +. But yeah, an easy DIY project.
Someone needs to invent a motorcycle helmet with a “ cabin air filter” ;-)