I just wanted to try to clarify a few things, according to my understanding.
Tiers are based on usage; they're also based on the power of pokemon, A.K.A. how much they centralize the metagame. Really, the only purpose for Uber and BL initially was to ensure those pokes wouldn't ruin the metas of the other tiers. The only reason Ubers evolved into its own meta is because it was able to gather enough members, and people still wanted to be able to play with those pokes. Right now the only examples I can think of are P-Z, Cress, and Garchomp.
Long ago, when Gen. IV came out, everyone thought Cress was Uber because they took one look at her stats and figured she was impossible to KO. But then once they realized that moonlight was cut down with SS, and she could only take so many crunches, pursuits and the like, she was delegated to the bottom of OU. This is where usage applies. She was not used too much anymore, an indication of her power level being low enough to possibly be allowed into UU. But there, if you were a witness, you would have seen how with a simple sub + cm set she took care of business. This is where centralization/power comes in. She was deemed too strong, because every team was forced to prepare for her well otherwise they would face one in literally almost every match and thus expect to get messed up. She didn't even require any support, because she could come in on practically any old resisted attack and set up in the opponent's face. Same for P-Z and Garchomp. They were all sent to their respective banlist tiers.
Now I've already said things about Poli and Ninetails in a thread about the former, but I'll sum up the points for y'all.
1. There are fairly equal arguments on both sides.
2. Shut up, stop theorymoning, and wait for the derned meta to develop. I don't care how much you think infinite rain/sun is broken, how weak the two bringers of insta weather are. Be quiet and be patient, that way you can brag about how you were right in the end. If you're trying to prove it now, you only make yourself look that much less believeable and are hurting your own cause.