This whole FP is 'better' argument is getting a little tired, better means 'always better' and that is not something that I consider proven - for now.
But keep in mind, you are hearing this from people that don't know anything about FP. The person who posted the Ask HN admits as much -- "I hear FP is awesome, but I don't want to learn it. Will someone please rehash at least eighty-nine reasons why I'm missing out? Because if there are only eighty-eight reasons, I am going to smugly declare that FP is worthless."
Let's face it, there is a such thing as a dumb question, and this is one.
There are some very adept FP programmers right here on HN that do not think that FP is a silver bullet, and there are a few that think it is, including PG.
I'm reserving my own judgment until I've gotten good enough at it that I can rightly say I can make up my mind by myself without external input.
That point is probably a long way off, but any attempt at ramming something down my throat (be it IP or FP is besides the point) is met with resistance.
Arguments, not rhetoric persuade people.
If FP were obviously better and as easy to learn as some claim then these arguments would be superfluous, the fact that it isn't means that the case is not as clear cut as some make it out to be.
Let's take HN as an example.
It's a site built by a man that is considered to be God himself when it comes to functional programming, working in a language that he himself states is going to be a 100 year language.
The site suffers regular outages, has serious limitations with respect to functionality that is present but simply does not work and small changes are apparently so hard to add that it takes months for even the simplest items. The way to deal with load issues is by cutting functionality (see the length of the per user comments list, new pages and so on).
Mr. Blub himself would have fixed these things long ago and would not tolerate such a state of affairs, programming in visual basic .net if that's what it came to.
If that's the 'poster website' for FP then I think that I'm either missing something or FP isn't as good as it is cracked up to be for the environment that most of the people here are working in (web programming).
Possibly a port of HN to a more stable platform (say clojure) would remedy some of these issues, but if PG can't 'hack' it then who am I to even try.
Arc is not really FP; it just happens to be a descriptive DSL for web apps.
news.arc has its limitations, but it's not because of functional programming; it's because pg tried to do as many things "differently" as possible (in-memory data-store, no "objects", etc.). The goal of news.arc, I assume, was to play with ideas, not to build a website. The website is just a side effect.
I won't debate the merits of news.arc's design, but I will note that not everything is about "building a scalable website". There is more to life than the output of ab.