Ok, I'm with Kiddu on this one. You guys are grasping at straws now. Being compact has never been a stated requirement for rapid spin. All that's been stated has been the ABILITY TO SPIN. While Steelix might not be able to spin the same WAY that most Rapid Spinners spin (horizontally) it's perfectly capable of spinning along it's length and smacking into the opponent and removing hazards. We even see examples of it in the anime and a demonstrating of the fact Steelix's body can rotate in PBR. So flavour wise, it's makes about as much sense as most Pokemon moves do and more than some.
While we're on the topic of Rapid Spin, what about Ferroseed and Ferrothorn? Hell, Ferroseed is constantly spinning in its in-game animation. And I really shouldn't have to explain to anyone here how they would benefit from the move.
And another topic, Fly! Why is Pidgey capable of carrying a fully grown human being but SCYTHER, who is big enough to fucking ride on, isn't? Or any of the other flying bug types for that matter? I've never understood Gamefreak's reluctance to giving flying bug types the ability to carry people...