I generally think it's legit but it's worth to remember that there is no silver bullet. It's important to have an exposure to multiple programming paradigms to be able to pick the best tool for the job. It's also important to always strive to write clean and simple code. Functional programming generally encourages it (for example by avoiding mutable state). On the other hand, some of the most difficult to maintain pieces of code I've ever seen were written in a functional languages. There is no language that can prevent developers from creating a mess.