I'll bite.

Why?

Being "functional" and using "Scheme for all stuff" is just a means to an end. What value does it bring me as a user or administrator?

Because I can think of at least one obvious downside: Scheme has no mainstream adoption. So they've picked a niche language (yes Scheme is niche) that users now have to learn.

So the value it brings, in terms of features, maintainability, etc, better be significant in order to justify that pain.

And I'm talking real value, not the value of intellectual rigor or functional purity. I'm talking about actual, real, tangible features that can't be gotten anywhere else.

Most sysadmin operations are non-destructive. This has huge implications.