Yes, taunt may temporarily stop Skarmory, Forry, and Hippo, but all they have to do is switch out and then switch back in, or hit us with a STAB Brave Bird/ EQ while we do nothing(Skarmory) or insignificant damage(Hippo). Really, the only way this could win against Hippo is to give it good Sp. Atk or give it reliable recovery, which would be kin of weird on an offensive pokemon. Sure, we can spin the hazards away, but that is a pretty much wasted movespace on CAP 9 and it would just be a never ending stall war. I think the only way to completely stop these secondary users is to get rid of them, as just scaring them off means they will just come back later.
I've already acknowledged that killing is easier. But, those are probably the biggies for this CAP. A switch out is a free turn for yourself. We can't assure ourselves that this will do nothing in front of Skarmory or Hippowdon either -- we have no idea what this thing will get, or what stat spread it will use. Rapid Spin is by no means a "waste of a slot" if CAP 9 needs it to Stop the Secondary. Dark/Ground makes an excellent typing to fend off Rotom-a, the biggest spin blocker in OU. Apologies for the minor poll jumping. We do not need to *win* against all of the secondary, although to cover most of them is preferred. Considering that this CAP is SOMEWHAT offensive, I think emphasizing one attack stat would be fine. There's also the Metagross example I posted above.