There are a couple of good reasons to use an air filter like the one in the photo. The engine is running for maybe a few minutes with the model on the ground what with engine starting, tuning, taxiing to and from the takeoff strip. Not to mention the occasional nose over that stops the engine. The air itself is full of dust and insects too. Another reason is that the filter traps any fuel that tends to spit back through the carb at lower revs so that helps keep the outside of the engine cleaner too. At about $4 and a few seconds to fit it's cheap insurance.