New slow riders slow you down, so you can't carry enough speed to clear jumps. That ruins the experience. The goal of a jump trail is to hit jumps. If everyone has a $10,000 bike and is clogging up A-line when they don't have the skill to clear a 3' gap, that wrecks EVERYONE else's ride who can hit the 15' gaps on the trail.
Those same new riders don't understand trail etiquette, and don't move over fast enough when someone calls out from behind them, ruining that guy's climb. So that's now both up, and down the mountain that the crowds have wrecked the ride.
Know how I avoid that? Weeknight rides, night rides, riding when it's crappy out....I have expanded the sandbox I'm playing in, not just sunny Saturdays from 10-2. I deal with less crowding this way.
The analogy isn't bulletproof, but it's not as bad as a bunch of people who don't understand mountain biking think it is!