The afks won't be an issue in Destiny as they were in Reach's firefight, but I've played co- op campaign enough to know players will grief other players and they will do it at the most inconvient times. So I suspect someone will be camping a sleeping Hive army and pop a round into them when you're trying to stealth out of a situation or someone will be running in with a monster train behind them yelling choo choo.