I think there are quite a few closet "functional" programmers, who may not even consider themselves as such, doing their best to write mostly side-effect-free,

I think there are quite a few closet "functional" programmers, who may not even consider themselves as such, doing their best to write mostly side-effect-free, declarative code within the confines of an OO or multi-paradigm language (in .NET land, functional-ish features like LINQ and pattern-matching bleeding into C# makes this easier). I also met more than one person trying to introduce F#/Scala in a pretty small .NET shop I used to work for.