I can't speak for everyone, but I rush to the boss because I've done the strikes over and over and over. I don't dash through the middle though. I go along the edge of the rooms to draw them out of the middle for my matchmates. By the time those two make it to me I usually have the boss started and close to done. I save the boss for them. That's a free run for you. When you've done the thing time after time, you get bored of farming them. If I wanted to farm I'd do it on patrol. The only time running ahead is a failure is when you're doing Nightfall Strike or the Vault. Get over it. If you for like that then back out and find another group.