Under cover

I've taken off the under-spoiler on my WRX a few times when I was road testing after an oil change. I'm not sure what the problem would be other than maybe a rock to the oil filter or radiator. They probably reduce drag to a degree and maybe even prevent damage on some driveways.

You're talking about those little two-piece plastic clips - right? I've purchased a few replacement ones, and new ones aren't deformed. They'll easily insert in the hole. A previously used one takes a shape where it has to be jiggled in about halfway. If it's inserted too far it won't close.

Don't know when they started it but my '03 has a small port that opens to get at oil plug and filter. Same for wife's '08. Remember it was a PITA on my '98 and I never took the whole thing off - just enough to change the oil. Never considered taking it off as I go minor off road through mud and fields.

They sometimes aid in cooling by directing airflow if the grille has a lower opening, under the bumper.

I Just recently replaced the one on the wife's 03 Outback. It was torn in one spot and the slide out access was also cracked. I dunno how much aero effect or protection they give, I guess i just couldn't tolerate knowing the car was incomplete LOL! Got mine from jamie at subarugenuineparts.com but, if you can make your local dealership match the online price, you may save money over the shipping cost. If you do 'serious' offroading, some of the guys at ultimatesubaru.org use metal ones. It 'seems' like it should be called a skidplate but I THINK Subaru calls it an engine undercover or something weird like that.