I think air cleaners are a good idea, but the only one I know of is the Bru Line which extends into the propeller on many engines. The best thing to do is not taxi your plane around any more than necessary. Carry it to the flight line and take off. When you land, kill the engine and go get your airplane. The longer your plane is on the ground with the engine running, the more dirt and sand it's sucking in.

I live in Florida so sand is a real problem down here. If you live in a place where there's real grass and good top soil, I doubt it can damage the engine much if a little gets sucked in.

Most guys don't use air filters and don't have problems. I like to use them whenever I can just because I'm anal-retentive.