And why does this answer blow it open? It seems like a slightly reactionary response to a pretty reasonable question to me.
I don't think anyone's in any doubt that purely functional programs simply aren't used in large systems, for most definitions of "large". There are counter examples (Erlang, EMACS, Jane's Capital) but they're currently the exception. I think it's totally reasonable to ask why this is, and how we might change it - either by making functional languages more accessible, or by promoting functional programming concepts in mainstream languages. It's totally possible to write largely referentially transparent programs in Java, for example, but most people don't do it - partly because the language and its idioms don't promote it, but also because the average Java programmer hasn't been exposed to the concepts. Maybe the "average" Java programmer would struggle to understand them, but a lot of them wouldn't. This second group could go on to promote functional programming, either in a more functional language or in Java - either way this is good, I think. Dismissing this sort of question out of hand doesn't help this process.
I don't think anyone's in any doubt that purely functional programs simply aren't used in large systems, for most definitions of "large". There are counter examples (Erlang, EMACS, Jane's Capital) but they're currently the exception. I think it's totally reasonable to ask why this is, and how we might change it - either by making functional languages more accessible, or by promoting functional programming concepts in mainstream languages. It's totally possible to write largely referentially transparent programs in Java, for example, but most people don't do it - partly because the language and its idioms don't promote it, but also because the average Java programmer hasn't been exposed to the concepts. Maybe the "average" Java programmer would struggle to understand them, but a lot of them wouldn't. This second group could go on to promote functional programming, either in a more functional language or in Java - either way this is good, I think. Dismissing this sort of question out of hand doesn't help this process.
See also: http://prog21.dadgum.com/41.html - this blog has a lot of interesting material on this theme.