Well Tentacool indeed ranks pretty high for its suckiness level right now. We might have to wait for months+ for the top Poison-type, Amoonguss, to get kicked upstairs, if at all. But if it does, then I dare say Tentacool may actually be (a tiny tiny bit) useful. So we're left with two options: one is just leaving it as is because no one will look at it (and we have to not take the effort of putting it back in when it becomes tiny tiny bit viable). The other is deleting it and later adding an analysis 'if truly necessary' (no matter how unlikely this is).
I'm fine with doing the latter. I don't mind a tiny bit more work, and we might not have to do any regardless. I'd theorymon that even if Amoonguss is gone then Vileplume will pick off exactly where Amoonguss left.
Still, remember that almost all Poison-types people use are restricted to Amoonguss, Skuntank, and Garbodor. If the opponent doesn't carry them, they are probably very weak to Tspikes. Of course, Tspikes are not going to be super effective because people will take simple measures against it if they get beaten a couple of times. Poison-types relatively don't suck that hard in NU. It's not effective unless it's a surprise 'back-door' strategy. Still, Tspikes can really catch your team unawares, and even a single layer can really, really suck.