They're part of the standard and don't cause any issues other than minor annoyance to that OCD demon in the back of our human minds when reading through code that contains them, at least as far as I'm aware.
Leaving them in makes it easier to adjust array and object literals if you're copy / pasting things or using keyboard shortcuts to delete lines or enter newlines (vim's dd and o for instance).
Only thing to remember is that they're invalid in JSON.