I agree with most above me, that Inconsistency turns a match into a luck based game, but even more than that. Unless the Inconsistency pokemon (me talking about octillery) has an attacking move that is only water type, it is ridiculously hard, if not almost impossible to beat, if luck doesn't fall your way (which won't in almost all cases of inconsistency). And even then, an Octillery with toxic takes care of that. Even what would normally be considered a hard counter can't take it out.
Natorie for instance. I run a Specially Defensive set, resistance to surf, not to mention he has tremendous bulk. However, I came across an octillery w/ Surf, Sub, Protect, Toxic. (i believe that was the set). Well, it can't toxic me, and I resist surf, not to mention my Special bulk, I figured no problem, especially since I have STAB power whip. But with octillery's combination of sub+ protect eventually got his defenses high enough that my SE, STAB Power Whip wouldn't even break his subs! And he eventually killed me with his surfs since his SpA eventually got sky high as well.
Does that make me a less skilled battler? In any other case Natorrie would have no problem taking on octillery. To tell me I HAVE to carry a vappy or suicune on my team, and HOPE that he doesn't run energy ball is ridiculous. Inconsistency gives the user an advantage so great that it becomes nearly impossible to defeat in many cases. I ALSO run an Unaware Quagsire. Still toxic-stalled, not much I can do.
I can't think of a single pokemon that isn't uber that can unquestionably take on Inconsistency Octillery. And if you about those of you who don't want to ban it "because pokemon should be banned only according to power", take this into account: Salamance was banned last gen. Without access to Dragon Dance I think it's safe to say he wouldn't have been banished to ubers. But he could set up. And when he set up he was monstrous. Inconsistency allows the pokemon to set up while protecting, or subbing! He doesn't have to waste turns DDing or SDing. His ability allows it to set up.
So yes, it may be slightly luck based, but it isn't luck in the sense that "there's a 30% chance to flinch", but that "there is a 100% chance you get a +2 boost to one stat, and -1 to another". And with 6 (i believe) stats that can be upgraded, and only 1 that is useless to octillery, that means there in a sub + protect combination, the odds of him getting +2 boosts in 2 desirable stats is 25:36. That's WAY more than half, meaning it isn't really a coin flip at all.