I get what you're saying but I'm still not swayed. Your example is pretty much limited to only situations in which both teams are running heavy hazards and one runs tentacool as a hazard / spinner(-er). A large majority of teams use only Rocks for hazards because both Toxic and regular spikes, though deadly when done, require way too much time to set up and (as has been mentioned many times) the giant list of amazing spinners vs the significantly smaller list of usable ghosts. What im getting at is that most teams wouldnt switch in frillish to a Tentacool. Losing Rocks > letting you set up. Frillish can't really hurt tentacool in anyways, and tentacool can giga drain, or toxic spikes. In fact the only thing frilish could do when it switches in, is block a spin - and then its immediatley forced out. When you have 6+ layers of hazards, spin blocking is incredibly important. When only rocks are on the ground, spin blocking is much less important because its so much easier (and plausible) to get those back up. Additionally, tentacool is not as common a spinner as Staryu / Drilbur. Of the two, Staryu is the bigger issue because it carries a Thunderbolt to peg Frillish. Drilbur has Shadow Claw though its hardly ever ran - favored for Stealth Rock, Swords Dance, or Rapid Spin. Anyways; in the case of staryu, double switches become more plausible, if still a generally bad idea. It boils down to prediction, switching into a thunderbolt is never fun, but if you do switch into a Spin, switching into a staryu check immediately after is significantly less punishing than with tentacool - who gives your whole team poison to deal with. Also, as Chielee menitioned, staryu's Tbolt *can* be out-stalled with recover, but not if it switches into it. Max HP/ Max SpD will only take 11-13 damage, which means you CAN switch in and outstall staryu, but leaves you significantly weaker physically. Lemme know if this needs to be clarified, I did my best not to, but I feel like im rambling.