Be the change you wish to see in the world (of Eorzea).
The players that are single-targeting a group of mobs might not know any better. And some percentage of those players will actually listen to constructive criticism. Then in the future, those now-enlightened players might help out the future generation.
Of course, there are still that other remaining fraction that will refuse to learn or listen to reason or good advice. Not much we can do about them. But the way I see it... I can't tell you the last time any party I had in DF failed to complete a duty. Maybe LotA back during its heyday. So instead of a 15 minute clear of a dungeon, it is a 20 minute clear. Move on.