The Engineers have found that some vehicles experience negative reactions with some systems in the car when the Cabin Filter is added needlessly. It is recommended that if your car did not come with one, don't add it. I can't tell you why, but one vehicle experiences cooling system problems when the filter is added. Just a thought, but these guys have a ton of education and experience and most of us don't know enough to override their designs. Hope that helps a bit.
Ken
TRD Toyota/Scion Parts Accessories from Sparks Toyota/Scion