yeah I do use them, specially against my classmate, who has only one legendary so far, so after we challenge with our normal pokemon, we decide to have one legendary in this time, just to make things a little bit harder to him, and easier to me, because his legendary is Yvetel and mine is Xerneas ( I have Yvetel too but not using it lol, I even gave him Xerneas after our last battle, so next time I must be careful which one to choose ;p)
so YEAH I am definitely using my Legendary when I am battling my classmate, but I don't online, because it wouldn't be a fair fight.