Languages come and go. Many development companies don't rely on just JS or just PHP or just Ruby. They generally need a combination of many for different projects. They may be supporting legacy code; they may work for different clients; they may just want to keep their options open.
One of my last jobs, while there, I wrote in C#, PHP, JS and Python.
It's good to be great in a language - PHP has always been my strongest. But don't lock yourself into 1. Get good at your primary language, spending a few years building and focusing on it - then, expand your horizons into other languages. It'll make you more marketable to companies and if problems arise, you can help solve them on multiple fronts.