Arceus has a lot of potential, especially as more powerful Normals come out of the woodworks (we already have a couple 70+ Normals to benefit from his effect). Double Normal procs about as often as Sinister Power from what I've heard, but the multiplier for the damage is 2.5x to Sinister's 1.5x. This means that in a timed stage that has an SE team but no 1.5x damage-boosting abilities that persist throughout an entire combo like, say... on a Fire Type like the M-Blaziken competition coming up next week, a Double Normal team has an edge over an SE team (even one with a 1.2x damage-boosting ability) if you're confident that you can keep the combo going with Double Normal in effect.
Of course, Arceus's effect is still mediocre in move-based stages. But in timed stages against Fire, Water, Fairy, Electric, Flying, Bug, Grass, Ice, Normal, Poison, and Ground... you might find yourself with a force to be reckoned with. Regigigas and Slaking are still somewhere out there, biding their time... somehow I failed to see the irony as I typed that last sentence.