I tried this notation in Chrome and it is valid.
Simply because it works in Chrome doesn't mean it's valid. It is valid because the spec says so :-)
I wouldn't use this notation but why would people use it?
To make copy&pasting easier. You can just append new properties without caring additional work. It's a bad practice in program code because older browsers (notably IE) and the ES3 spec disallow them, but in a config file (i.e. in a known environment) it makes life easier.