It also seems straight up dangerous, for themselves and others, at least on downhill trails. If they're going down them, well, it's already dangerous enough for beginners (who always seem to ignore the difficulty ratings) without adding extra speed. But the issue I've seen time and again is folks riding those things up the trails that people are actively trying to ride down. They were never designed with two-way traffic in mind, and going up it just chews it up even more.