It isn't about rocks, it's about having your engine ingest dust, dirt and grit which destroys it's ability to seal the valves and rings. Which of course shortens the life of your engine. How stupid is it to build an high performance expensive engine and then destroy it because you are being stupid not running an air filter system. Hell, even the NASCAR guys run a filter. Then again, you can fix stupid. But fortunately; "Stupidity, if left to it's own devices, is almost always self correcting"