Here are just some of the observations I've made about Baton Pass users in Gen 6 UU. There is no true conclusion I'd like to make from them, since with regards to tiering, I don't have a solid stance on this surprisingly complex move. This is just for anyone's reference (and mine, too).
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This wasn't the first time I've seen Baton Pass heavily influence the metagame. There weren't any infamous BP chains like the ones seen in OU (especially since Espeon dropped both after Scolipede got banned and after one of the older forms of the BP Clause halted this playstyle). Instead, Geomancy, Venomoth, and Scolipede demonstrated what the move could do to tear the metagame apart when stats were passed.
Last time I checked, though, Baton Pass was
not the reason for the success and popularity of other Pokémon (that learn the move) that shifted to OU or BL. Togekiss, Hawlucha, and Mew had other prime movesets and qualities going for them to succeed in this tier. Mew in XY UU did Baton Pass stats once in a while, though, and Celebi used Baton Pass about as often as it does today. While they proved to me that Baton Pass was a powerful tool, they never struck me as powerful as the likes of other moves like V-Create.
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The current Baton Pass Clause was enough to both unban Geomancy (man that was such a long time ago) and get Venomoth to be a stable Pokémon in UU. The Clause didn't save Scolipede (SD + Speed Boost w/o Baton Pass is so good), but it did allow the likes of Celebi, Espeon, and Sylveon to live in the metagame when they dropped from OU.
[I wonder...if Mew had dropped back in by usage, would people still sleep on Baton Pass Mew and direct attention to the Stallbreaker set or SD/NP+3 Attacks like before?]
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Between Celebi's time in OU and its return to UU, Baton Pass saw itself among the tier's Pokémon as a viable option for Espeon, Zapdos, and Shedinja. That's not to say other Pokémon didn't use the move: it was a decent niche option for Vaporeon, Umbreon, Gligar, Mega Absol, Durant, and Togetic.
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It's kinda nice to see Chicken Pass succeed in this tier for once. I always wondered what it would take for Combusken to put enough points on the board to earn itself a possible viability rank.