I can't answer the first question since I'm relatively young and didn't need much convincing to join. I just figured it would be a natural step in my learning curve and a great way to meet like minded people.
As to the second question, my initial impression was to say that a show isn't necessary, but after thinking about it, I'm not quite sure. A koi show probably represents the single best avenue for koi appreciation and education. Isn't that what a koi club is all about?
-Dan