I often hit the "warp out" button when I see someone working for the optional objectives instead of the mission completion. It's completely selfish and since i do not approve of that i'll never help that kind of player.
The only reason why people bother doing the mirror event is to farm XP, that's just the nature of the beast. Don't like people leaving as soon as the Stadi shows up? Don't queue for mirror events; you can only control your own actions, not the actions of others.