To play devil's advocate for a minute:
Nearly all useful, reusable software today is written in a mostly or entirely imperative language. This is despite the fact that functional programming has been around for at least 20-30 years. So, my basic question is, if functional programming is so much better, why isn't more software written in a functional language? Or put another way, why are there so many blog posts promoting functional programming when it clearly hasn't produced results.
That's a good question and a proof that pure FP isn't good for creating "stuff that works". For years the mantra was "bad industry doesn't want to use FP". But nowadays, especially inside a community like HN, this statement doesn't make much sense. There are plenty of side projects and startups, and how many of the successfull ones use pure FP?
I think the reason for this is that pure FP replaces classic programming with puzzle solving. It satisfies math-oriented minds by forcing usage of this math-oriented layer. But it looks like software isn't math and you often need to work with a computer in a more direct way.
For example, the currently highest-voted comment on /r/bitcoin for the story [0] says "I disagree with him [mitsuhiko]".
[0] http://www.reddit.com/r/Bitcoin/comments/2kz9x0/please_remov...