Want to develop web applications? Nothing from the functional programming camp comes even close to the polished experience of using Ruby on Rails. Yes, I know about Play and other web frameworks for Scala, Clojure, and Haskell, but - and I repeat - they aren't even remotely close to the level of support, libraries, plugins, and documentation that you can get from the Rails world.
Want to develop mobile apps for iOS or Android? That's typically Objective-C or Java territory. Yes, you can develop Android in Scala, but you are a pioneer then (and you are virtually on your own if you encounter any issues with the stack, as very little documentation and community support is available for it).
Want to develop desktop applications? Again, functional programming languages are rarely the best supported option. C#, Objective-C, C++, or even Object Pascal (Delphi) are all fine choices. Opt for Scala, Clojure, or Haskell and you'll find little documentation and support, and in the case of the first two, you'll have GUIs that look anything but native.
So what are these functional programming languages good for? So far I've only found them to be good at doing heavy lifting in the back-end or in GUI-less scripts. An important spot for sure, but not the most prominent one to justify adopting either as my main tool.
I love functional programming languages and I wish I could drop everything and write web apps in Clojure or Haskell. But the hustler in me wants to get stuff done and cannot justify the world of pain and waste of time I'd experience, especially when there are already tools that are simply better suited for the task at hand.
Should a project of mine scale to the point of needing a fast functional programming language for core functionality in the back-end, I'll be happy to employ it there.