> that will really drive home the importance/benefits of functional programming..

From my experience with Haskell, on a practical level, I think it really comes down to a guarantee that you will not be able to mess the code up beyond a certain extant.

I am not really sure if it should be attributed to the type system or to functional programming in general. A functional programming language require a really good type system, because you need it maintain sanity with you have to chain together functions a lot more than you do in an imperative language. So I think it has to be a mix of both.