I have been in several discussions with friends and colleagues about the limitation of OOP and virtues of functional programming. It seems as we push harder for code testability and lower bug rates, it's inevitable that everyone will eventually find the ideas of side impact-less, perfectly enclosed functional programming appealing.
Yet you rarely see business/web applications written in functional languages, and it's not because developers are all lazy.
See, the ultimate goal of almost all software is to solve a problem, it's never about making the most architecturally elegant solution and the most beautiful code in history. And when you focus on how to solve the problem, without worrying about the language, framework, or the programming paradigm you are gonna use, the solution you discover will make most of those technical decisions for you.
When you decide up front that you are gonna use a certain set of tools to a problem you don't have solution yet, you are essentially handicapping yourself, and in many cases will end up with wrapping functional logic with an OOP layer or vice versa.
In short, when you are solving a Sudoku, focus on the god damn Sudoku, worry about unit tests later.