My opinion on the matter? I was never a fan of collateral bans. Sure, it's the simplest way to deal with it, but is it the best way to deal with an issue?
Basically, I'd say to ban When the Ability or Combination breaks all relevant Pokémon that have it.
I understand the whole "swift swim luvdisc" argument, but that just doesn't work. NU Pokemon like Luvdisc, in this instance, should not be considered in making any sort of decision as it's just not used enough to really matter at all. Usage is important here. If something isn't used at all, why ban it?
As for bans, they need to be enforceable, discrete, and warranted. The first two are easily done, as we can tell what Pokemon is getting banned as it's clearly defined. The problem is whether or not it's warranted.
First off, whether or not something is "too good." It should only be called that if it centralizes a game to the exclusion of almost everything else. Case in point, look at the early DP metagame. Teams basically consisted of Garchomp, a counter, a counter to said counter, etc. That's a perfect example of when something is too good for said metagame.
In other cases, just because a certain ability or technique changes the game does not mean it breaks the game. Even though things like Reuniclus and Excadrill are potent threats, for example with their abilities, they're still perfectly counterable. Things like Moody/Inconsistent are not.
Consider how Inconsistent basically turned the game into a game of chance. If left unchecked, it'd get to the point where people would need phazers, Perish Song users, etc. This would be fine and dandy, except said ability broke even the irrelevant Pokemon.
Now, as for Speed Boost Blaziken. He did get banned. But, does the ability break the likes of say, Ninjask? Yanmega? Sharpedo? For example, Speed Boost is all that Ninjask has, and even though he can use it with Baton Pass, he's still incredibly frail and is limited by his environment with phazing, Taunt users, etc. Yanmega, while an excellent user of the ability, has extremely hard counters. Blissey can stop it, and wall it to no end or force it out. As for Sharpedo, he's not as common of a threat, but even then, he still can be countered. Priority kills him, and he cannot take a hit to save his life, alongside many things being able to take a hit. I'd only argue for the ban if it is absolutely warranted and overcentralizes or breaks the game. Just because it's highly effective against a playstyle does not mean it's broken.