The correct answer: "No."
Yet another Pokemon Trainer has requested that everyone bans themselves from using Legendary Pokemon in online Pokemon Omega Ruby and Pokemon Alpha Sapphire battles.
Here we go again... just because a Pokemon is classified as "Legendary" doesn't automatically mean it's overpowered; for example, Aegislash and Blaziken aren't "Legendary" Pokemon, but they're classified as overpowered Pokemon.
Besides, asking for total ban of Legendary Pokemon is ridiculous, considering those Pokemon are... Pokemon. Correct me if I'm wrong, but aren't all Pokemon meant for battle?
I personally have no issues with other Trainers using only overpowered Pokemon, because that allows me to test my Trick Room strategies. Watching my overpowered opponents rage quit over my Clawitzer, Cofagrigus and Ursaring opening up a can of whup ass is satisfying.