The only uses I've found for cover so far are in the bavarois fight in Wanderer's Palace (you can cover one of the party members when the boss randomly targets it, though cooldown only lets you do one. It still feels safer to just run though) and perhaps better, casting it on a melee during the Livia Sas Junius fight in Castrum Meridianum. As long as the melee doesnt -run- you should be able to mitigate all of the damage from the adds targetted on them and you can just pop a cooldown and let them dps as normal.

Neither of those fights are especially tough though so it's hardly needed. I would definitely use Cover more if the range wasnt so restrictive. I try to use it as much as I can but everyone just gets scared when bosses have random target periods and runs away from me.