> I'd encourage him to build something so awesome with a functional language that the rest of us can't help but be blown away.

Functional language helps a lot in building of side effect free code. But such code can be written even in pure assembler. I writing in such style in any language, including Bash.

Sometimes I need to refactor somebody else code. Code functionality remains exact same after refactoring. Things that changed are: better code clarity, better reusability, and better testability as part of reusability.

Unit tests creates additional contexts, in which main code can be used, thus they force to improve reusability of the code, thus code forced to make less or none side effects, thus programmer forced to rewrite code in functional style.