As for the Cloyster, it's one of many Pokemon who have emerged from complete obscurity, all because of Froslass. That's called overcentralizing if I remember correctly.
The thing is, overcentralization is if you use something only to check one thing in particular, meaning that the rest of the time it's dead weight on your team. For example, this does not mean that if you throw something like Torterra on your team solely to check Registeel, that's overcentralization, because Torterra, used right, can be effective against many other Pokémon as well (Rhyperior, Slowbro, and Aggron come to mind).
Similarly, Lemmiwink's Cloyster is not dead weight on his team - he's said before, and I believe him, that Cloyster serves a useful purpose on his team, thus its inclusion. Before Froslass's inclusion, it may not have done enough to warrant a slot for him, but its induction finally pushed it over the hump, and many battlers have a good use for it, where it is not outclassed by other Pokémon (Blastoise, Omastar, Qwilfish).
Honestly, we haven't seen any real overcentralization yet. Actually, I'm kind of disappointed in this early testing period - the whole metagame is radically unstable, and it feels like January all over again. But that's my opinion - I just haven't been to adjust yet.
Ambipom is EVERYWHERE. What's up with creating anti-Froslass leads, when they can't handle Ambipom (Read: other Ambipom)? In all this hubbub, one Pokémon has stood out to me as an effective check to the current Top 2 Leads - Spiritomb.
Ambipom can't do anything to it, neither can Froslass. On the former, just spam Pursuit, the latter, Pursuit + Shadow Sneak.
