Well, it's a lot easier and more intuitive to program with side effects, for one thing. Functional programming is difficult for a lot of people to wrap their head around -- find someone who's taught/TAed a class in Ocaml and you'll probably get all kinds of stories about people's abject failure to comprehend it. And what good is having beautifully designed, wonderfully side effect free functional code if nobody can actually follow it? Makes hiring people to get your software done rather difficult.
That's one side of the argument, at least. There's any number of reasons lots of people are going to have to learn all about functional style, side-effect-less code. Multithreading comes to mind.
