Why don't they just take all the mobs away.
Q up, enter dungeon, click on treasure chest & done.
That would solve all the problems now wouldn't it?

I'm not sure which is worse, players trying to justify their laziness or SE condoning the behavior.

If it were up to me there would be no leaving a dungeon and therefore no need for the 'join in progress' option.
If you don't have the time to complete the dungeon then don't q up.

I don't see people coming into a movie half way through, a football game until halftime, or a restaurant part way through dinner, etc.
So why is it OK here?