ac cooldown is fatal. ok, it's good for arena or for raid tanking when a base isn't well covered, but in coop dungeon best tank is pally. ac reflects damage taken but when it's in its cooldown in the meanwhile boss procs and ac takes damage, and when stun ends, druids still have to recharge (add this: boss reduces to 0 or 40 energy) and boss can easily take down ac.
with pally is different: he doesn't reflect damage, has a 1 sec cooldown but has a higher health and a faster atk spd, so if boss procs, pally will surely be using his skill, reducing damage, if 1 or 2 secs too, but what makes him survive is that when stun ended, pally can proc in less than 2 secs, that combined with his higher health (and maybe a scorch too why not), he'll take enough time to have duids proccing and keep surviving and taking on him every attack