After many oil changes, the plastic underpanel that must be removed from the 2005 Mazda 3 becomes pretty worn, particularly if the oil changer does not put all, or the right clips back in after every change. Is there really any fundamental reason to keep reinstalling this plastic underpanel? What are the ramifications of just leaving it off?

 

Some have taken it off and claim no damage. I chose to keep it but cut a hole under the filter just large enough to remove said filter. Many have done that also.
I ran w/o the shield for a while and found that the side panels will come into contact with the axles and wheels, so I reinstalled it rather than removing those panels. Many believe that it helps to keep the engine compartment cleaner, me included.

 

It improves airflow under the car and that's about it.