Well nowadays, all of the good rapid spinners are dead obvious. So once you pull one out, your opponent pulls out Rotom formes or Spiritomb to spin block you. Rapid spinner is a really limited technique that not many strong pokemon can learn, so its too predictable to be of any use (hence the reason why most teams don't even have a rapid spinner). By giving more Pokemon rapid spin, you're making it a lot more unpredictable and a lot more useful. Imagine if something like Bronzong could learn it. Bronzong is terrific for support and has a lot of uses (the only current rapid spinner that kinda shares that role is Starmie), so having Rapid Spin would be a wild card. Or if a Pokemon could learn pursuit and rapid spin, that makes it even better since you can easily dispose of any ghosts that come in your way and get in a rapid spin when they switch out/faint. Imagine something like Metagross being able to learn rapid spin (which, like Bronzong, it should in the first place). You can Pursuit/Shadow ball and kill any ghosts that happen to be there when Metagross switches in, then rapid spin, providing clearance for a BellyZard to switch in. And when you pit Metagross's bullet punch and meteor mash against Spiritomb, we know who wins. Rapid Spinning starmie w/ shadowball just to nurb those ghostly switchins :) problem solved :P