The only time you are at significant risk of sucking a lot of dirt into the engine is on the ground. If you fly from asphault or concrete, there is no need for a filter. Most grass runways are also typically clean enough that a filter isn't needed. I have heard of some that use filters becasue the site conditions warrant such, but in general, the vast majority of sites don't require a filter. One might think "why not just use a filter anyway." I have used filters before and they will rob power. I dropped 150 RPM when I used a filter on a YS140. 150RPM may not sound like a lot, but it equated to 0.15HP on that setup, or a 5% drop in power, just from the filter.