-Answer #1, #2:Originally Posted by Strype McClaine
Arceus Plates, like other items such as Charcoal, Magnets, Mystic Waters, etcetera, increase the damage done by 20%, as long as they correspond to the specified type. This is a 10% increase from what the items use to do in past generations.
Personally, I think they're fine items to have, but if you can manage to get either the Muscle Band (20% increase in damage done by physical attacks) or the Wise Glasses (20% increase in damage done by special attacks), take them instead.
Copy+Pasted from a "googled" post