User blog comment:Supp0rt4n00bz/Welcome to the League of Hydra!/@comment-1330314-20131219173311/@comment-9048637-20131219180902

Even in combat, sometimes Hydra > Bloodthirster just for the extra burst potential from the passive. It comes down to a question of which is more useful: the extra 25AD (assuming a fully farmed BT) on your standard combo, or being able to follow it up immediately with an extra 100% of your AD? The answer depends on the situation and the champion.

That's for assassins, anyway; on bruisers, Hydra is just indisputably better in nearly every situation because of the extra teamfight presence it gives.

(Also, as NeonSpotlight notes, Hydra's build path is way friendlier.)