This might be beside the point, but big pulls will ( if they aren't dense) force a newer tank to get used to the idea of mitigating damage through cooldowns on trash pulls. I can't tell you how many times in lower lvl dungeons I see tanks not using their defensive cooldowns at all until the boss. This is a huge disconnect and learning how to deal with big pulls forces a tank to use their cooldowns intelligently or the party will wipe. Sure, the first couple times you try your gonna fail and go down in a blaze of glory, but usually you can give simple advice about using cooldowns to their advantage and you can see a mediocre tank become a good tank over the course of a single dungeon.

Knowledge of proper cooldown rotation and pre-meditated use of the lvl 50 tank cooldowns is not taught in the game system. It's up to the player and senior players to help people along with this.