Fewer Pokémon having it means that it's actually possible to look at each case in depth and respectively ban the Pokémon if needs be. On the contrary, we couldn't do that with Double Team for way too many Pokémon had it and therefore banning them all would be ridiculous which is partially why it is exceptional. When something is broken, you don't ban part of that brokenness to weaken it and make him usable, you simply ban the Pokémon. Why wouldn't it be the same for Octillery? Plus, Inconsistent isn't as strong as you say it is; the Pokémons that get it are mediocre and wouldn't benefit so much from that ability that it would make them broken.
@ Above Way more than that get Double Team.
Ok then, what if half the metagame got inconsistent and we found it broken? Would you agree to ban inconsistent rather than the pokemon then?
Yes, but their effect being nearly the same, it would be like adding 8 more Pokémons to the crowd that learns Double Team.
It is still an Item. It may affect Latias/Latios only, but any Pokémon can carry it. It is simply an item Latias can choose to have just like choice scarf; it's not part of the Pokémon.
Incorrect. An item is every much a part of a pokemon set as a move or ability. While all pokemon share pretty much all items, there are a few with restriced item pools (mainly giratina and Arceus).
Maybe so, but you know the effect of Double Team/Minimize; it's in your total control whereas Inconsistent is not in your control you get whatever the game whats you to get or lose. It's the same kind of luck as in Acupressure.
You have no control over the effect of double team minimize; Their effect is to give you a chance of dodging moves. You have no control over whether or not it actually happens. The reason acupressure is not very good is that you are open to attack while boosting, while inconsistent pokemon boost by protecting themselves with protect/substitute.
Read my other answer; being nearly the same effect, Minimize and Double Team stack up.
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There is always a part of luck in the game: Crits, miss, etc. This ability is in no way broken for the Pokémons that have it aren't good in the first place - but that's not really what I ''care'' about. I'm just saying you can't ban an ability, you must ban the Pokémon along with it.
Have you even been playing lately? It's quite simply put ridiculous.
You can ban plain abilities, if they're inherently broken. We have banned parts of pokemon sets before- moves and items. The main requirement is that the rule be simple- have only 1 condition. No "No ability y on pokemon x"- it requires to conditions, just, "no ability y allowed." Indeed, the main difference between species and the other parts of a pokemon set (ability, moves, and items) is that a species is linked to a list of the three others, while the others are only linked to a list of species.