Sensationalist stuff. isArray doesn't depend on anything, but lots of other packages depend on it. Why? Because it's actually kind of hard to tell the difference between an array of things and an object with integer keys. The Array.isArray function wasn't added to the language until ES5, so you need a way to shim older tests.

If you're writing only for modern browsers, you don't need it (if you actually visit the code in question you'll see it defaults to Array.isArray - in that sense, it's a polyfill or whatever). But if your code might run on old browsers, it can't hurt to have it, and it cleanly encapsulates a trick that you no longer have to remember the syntax for.