Have a look at http://www.indeed.com/jobanalytics/jobtrends?q=c%23%2C+f%23&...
And someone has to maintain your near functional or functional code
You don't have to hire developers from outside to bring new technologies into a company. In fact, given the scarcity (i.e., demand > supply) of good developers these days, it's almost certainly going to be easier and faster to have your existing development staff learn those new technologies vs. trying to hire new developers with experience in those technologies.
I think I actually spent more time learning Entity Framework than I did learning F#, and EF still catches me with weird edge cases. Meanwhile F#'s crazy smart compiler is catching all mine ;)
Also, with default immutability, type providers, and less boilerplate I feel a lot more confident with the quality of the code I write, and I am able to write it faster.
Since it's possible to do full C#-style OO in F#, I personally don't see a reason to use C# at all. F# is more supporting of a multi paradigm codebase, it's safer, and requires less code to do the same things as C#. Combine that with what I would consider to be an extremely short learning curve, I am happy finding C# programmers and letting them learn F# the first week of the job.
Sure, it does, but why would you use it like that?
I like F# though. But I think it takes some getting used to, and I'm not quite there yet. :)