It is faster and more efficient to use AoE attacks on more enemies. If you are properly geared for the content and know how to play your job, mass-pulling is the most logical thing to do.

If a tank pulls trash into a boss fight, they are using the lockout technique. You can stand inside the barrier while the mobs are outside and pull the boss. When the lockout barrier appears, the mobs deaggro and you can fight the boss without needing to kill the trash.