The tanks are stuck with the though that "ohh i need to save it for when a big damage is coming or when my HP is low..." but in dungeons the most of the time the trash pulls are doing more damage than the bosses especially in large pulls.
Also even if they use CDs during pulls most CDs will be ready by the time they get to the boss so it should be that in dungeons the defensive cooldowns should be cycled and in raids and trials they are used strategically.