I hate to bring up the whole "philosophy v. pragmatism" argument again but that's really what we have here.
Point 1) Obviously, allowing a "ban" in Ubers based solely on power (and not luck/uncompetitiveness) would irrevocably destroy the philosophy behind Smogon tiering-- this is where the big issue is. Banning Arceus is absurd philosophically.
Point 2) We're talking about the simple in-out of a single pokemon in a circle of play the vast majority of us don't even play. Whether Arceus is allowed in or not has diddly-squat direct affect on OU, UU or any of their players. Speaking pragmatically, allowing the whole damn community to veto any decision by said circle about how said circle plays due to philosophy is equally absurd.
There are so many players in the community who just want to have chance to lead by the voice of pragmatism (just look at the "follow game mechanics perfectly" poll), and yet we as a community end up falling flat-faced every time we try. In the end, Phil had to play babysitter.
Frankly, it's embarrassing to eat dirt on this issue at every turn.
Fortunately in this particular situation we can make a convoluted back-hole to have our cake and eat it too. We just have to play with definitions a bit. The below is for those philosophical die-hards who can't just chill out and let people play.
1) Define OU and UU as "Standard Tiers" (or Standard Ideal tiers)
2) Define "Standard Tiers" as being both Skill Based (better players rise to the top) and lacking over-centralization (built to have a certain degree of variety)
3) At this point, Ubers is a ban list to make the "Standard Tier" OU. However, it is also played as a metagame by a circle of players.
4) Separate the "ban list" and the "metagame" as 2 separate entities, you can even give them different names if you want like "Uber List" and "Uber Meta" for instance. Uber list serves as our list for OU, and Uber Meta goes and becomes it's own metagame like LC or CAP.
5) Identify the Uber Meta as a metagame that is skill based but also overly-centralized. This is, in fact what Ubers is.
6) Thus, define Ubers as a competitive metagame (able to have its own rules, bans, tournaments, etc.), while still leaving OU as the standard.
7) Effectively, this allows OU players to do their thing and be the standard meta, while also giving Ubers freedom to make independent policy decisions.
It should be noted that this does affect what types of bans that both are allowed to make.
OU, UU, as standard metagames:
A) Can ban based on uncompetitiveness (causes reduction of skill-based play)
B) Can ban based on over-centralization (dominant to the point of reducing variety/balance)
Meanwhile, Ubers, as a game that is only competitive, but not standard, can only make bans based on A, not B. In other words:
A) Ubers can only make bans based on uncompetitiveness, not over-centralization.
This is in fact, already how things work. Ubers has Evasion Clause, OHKO clause, Sleep-Clause-- it has all the regulations based purely on A and not B. All of the bans we made from 4th Gen OU (with the borderline exception of Wobb) were heavily contingent on B.
Now, it's true that this still leaves interpretation as to whether Arceus can be banned under A. Under the above logic, it can be.
Arceus doesn't reduce variety, it increases variety. That's why it's shaking up Ubers so much (if I am reading Jabba's and other's posts correctly)-- it's dramatically increasing the number of threats in a metagame that has been built around limited options. Adding variety inherently adds luck to the meta (the more variety, the more luck based on team-matchup becomes a factor). This added luck can be argued to be reducing the competitiveness of Ubers, or as Jabba has worded his complaint: "turns the match into a crap-shoot." That's a complaint about reduction of skill, therefore a nomination under uncompetitiveness, category A).
The only things I have left vague and unanswered are what constitutes an infringement of A or B, what makes Double Team worthy of an A banishment while Discharge is not. What makes Groudon worthy of a B infringement while Tyranitar is not. The two terms are inherently vague here-- but this is where the judgment of the players comes in. The players themselves, have to decide what does and what doesn't infringe on A or B. This is why, despite any degree of theory and philosophy, all tiering decisions ultimately have to come down to subjective judgment-- to the vote system.
Follow the above logic, and maybe we non-ubers players can live and let-live and let the Ubers guys do their own thing in peace. >___> Let's get out of the way and let them make the decision!
Edit: On recommendation from Eric the Espeon, I changed the word "competitive" to the more concrete term "skill based"