I'm going to repeat my proposal that Cloyster should be in C-tier. There's just nothing too amazing about it that would justify a place in B-tier with other, more effective sweepers - even after a Shell Smash it doesn't quite hit hard enough to break through the common bulky Waters/Steels, and isn't fast enough to avoid being revenge killed by most Scarfers. It's also extremely difficult to set up as almost all Special attacks will OHKO it, and even some Physical attacks will do a ton after a Defense drop. It may be quite high in the usage statistics, but remember that it's very popular lower down in the ladder. I almost never see Cloyster used by any of the top ladder players - there's bound to be reasoning for that. However, one Cloyster set that does have great potential, that sadly no one uses, is mixed LO with Naive nature. It's fast enough to outspeed Scarf Terrakion, can easily OHKO defensive Steels with a rain boosted Hydro Pump and hits harder in general thanks to Life Orb.
If anyone has a solid argument for Cloyster's placement in B-tier, then I'd be delighted to hear it. Otherwise, I stand by my notion that it just isn't dominant enough to get a place in B-tier.