I think anyone who has to use more than one legendary in a battle team is too dependent on power handed to them; too lazy to train non-legendaries well enough to defeat whatever they face. I have a team made entirely of non-legendaries in Emerald that has devastated entire legendary teams of several friends of mine. I do plan to use one legendary, but it's not one of the many hulking 00ber sweepers everyone uses, it's Darkrai, who just happens to be a master of sleep techs.
So... yeah. I'll catch Arceus, but I'll carve up my DS before I use it in a real, competitive battle.