Ok, on the Wiki I keep adding Arceus to the type pages and people keep reverting it for, what appear to me, to be straw man arguments and I'm frustrated and annoyed by it. Below are links to their communications:
Various Pokemon have differing forms and different methods of achieving them, some change because of the weather (Castform), some by use of a move (Meloetta), some by a certain status (Darmanitan and yes, I am calling low HP a status for lack of a better word). Arceus's method is through the use of its plates and it is one of the only Pokemon capable of being ANY type, as I don't believe Mew or Staryu is capable of being ???-type.
So what is the problem? We are agreed that Arceus's plates are a method of form change, we have agreed that should a Pokemon be able to change type through forme, or even to be able to change forme at all (why Genesect is considered to be able to change form through cassettes when only a SMALL part of Genesect's body changes but nothing else, not stats, or moves it can use, just a part of it's body and ONE move, that it never gets STAB from. Keep in mind that if a Pokemon were holding one of Arceus's plates and managed to use Judgment, maybe through Metronome or Mirror Move Judgment would take on the type of the plate, not Normal or the user's type).
Arceus is UNIQUE. You can look to the Eevee line to see the usage of various stones and other methods to achieve most other types, but Arceus is one of a kind. No other Pokemon can change its type the way Arceus can, and if it could it would need to be added to those type page, too. Any Pokemon can hold one of Arceus's plates, but ONLY Arceus is capable of taking on its type (unless another Pokemon were to hold the plate and switch abilities, but that's rambling).
EDIT: BTW, I wonder if all of you have seen Arceus's page? http://bulbapedia.bulbagarden.net/w/index.php?title=Arceus%20(Pok%E9mon)&action=purge (be sure to hit F5 a few times)
EDIT: A list of logical reasons why Arceus should be added (contributors include me, Vuvuzela2010 and SnorlaxMonster on the wiki, 05308, Outrage and Nintendo, though not affiliated):
1. We're mostly all in agreement that it is a forme change
2. Unless there's really a good reason not to we should keep with policy and respect that it is a forme change that like the rest must be noted in its proper place; as this refers to type in this case Arceus's various types should be documented on those pages like all other forme changes that affect type.
3. Nintendo has programmed the game to even display different type images of Arceus
4. The plates do have individual names
5. Arceus can remain the type of its plate as long as it has it, in battle or out
6. They even gave Arceus alone this unique ability
7. Even if any other Pokemon were to take Arceus's plates the only use they have for it would be to boost the power of their moves that are the same type - Since no other Pokemon has the Multitype ability if it wanted to use the ability it would need to use a move that can swap abilities, and there's not a long list of Pokemon that can do this (@ 10, it's likely impossible) and it is pretty unlikely they'd even be able to get hold of a plate anyway.
8. Arceus's formes also affect its signature move Judgement.
This further adds to the uniqueness of Arceus's forme changing and further makes it even more difficult for any other Pokemon to mimic Arceus's uniqueness as a Pokemon would have to somehow A. attain a plate B. copy Arceus's ability and this can ONLY be done in battle. C. copy or get lucky enough to use Judgment at the same time. See point 10 and 11 for why this is not possible.
9. The ability is permanent and Arceus will have it in any game or the Pokemon Dream World
10. Various exceptions have been made simply for Arceus and its plates, for example the move Knock Off or Thief will fail to remove a plate. Skill Swap and Role Play cannot take Multitype so ONLY Arceus has access to this ability.
11. Arceus changes physically from interaction with a plate - This is something most if not all Pokemon with forme/type variance have in common.
12. Were it possible for a Pokemon to copy Arceus's ability while holding a plate itself it still wouldn't change color like Arceus can.
13. It's Pokeathlon stats change
14. The potential of its moveset is affected by its ability to go between types - It can only be tutored Draco Meteor when it is Dragon-type.