What's with all these people single-targetting during big pulls in dungeons?

[Conquistadork]

That would depend on the job and play style. As in my case, when I'm using SAM, I tend to do a one single combo on each pull to get my dmg up buff first and then proceed to do my AoE combos. It also depends on the type of mobs you are fighting, if there's one of those huge monsters that love to do AoEs, I will focus it first to remove it as fast as possible.

Some classes have better ways to deal with group mobs than others, so I see why they would feel more comfortable doing single combos, is it the best viable option? perhaps not, but you can't expect everyone to play the same way as you, or your FC mates do.