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Smogon tries to avoid complex bans wherever possible. That's why a pokemon can be terrible with 1 or 2/3 of its abilities but if only one works well on it then the whole pokemon's tiering is based on it.
But let's ask ourselves something for a second.
Suppose Blaziken never got Speed Boost, but instead got some other useless ability like Inner Focus. Where would it be tiered and why?
I would guess UU because it has very high powered Stab options, good boosting moves and Baton Pass. The mediocre speed can be worked around with Scarf or Agility.