I don't think you need to be a great spinner to teach the skills to a youngster.
The inherant talent in a spinner, IMO, is in the ability to spin the ball. Large hands, strong fingers/shoulders, supple wrists. You can't coach those.
But once a spinner has those qualities, he needs to be coached into using them right. Be taught how to make the most out of them. Stuff like optimising his action (not changing it) so that he releases the ball faster and with more spin, showing them the grips for variations, those are the technical things that can be taught by nearly anyone who has an understanding of spin bowling and the mechanics behind it; regardless of how succesful they were themselves at their level.
What makes a skillful spinner into an effective, and match winning one, is teaching them how to use these skills. That's where a guy like Murali is so brilliant. Teaching a young spinner how to read a batsman, encouraging them to experiment with their angles and flight, giving them a list of basic bowling plans to set up batsmen and giving them the inspiration to create their own traps. That is key.
But just as important as the knowledge, is the opportunity to try it out. Net practice is great for spinners, and more effective for them than fast bowlers IMO, because it allows them to keep bowling their variations and from different angles till they master all of them, and have the control they desire. But it's still no comparison for actually bowling in a match.
Because what seperates a good spinner from a great one is not the skills, but knowing how to use them in the context of a match. A lot of spinners in FC cricket around the world can spin the ball big, bowl a topspinner/googly/slider, land the ball where they want to most of the time. But not many of them know how to react after a batsman has hit them for a few big shots, or when their basic bowling plans have been worked out.
