Their site has a pretty bad example. Instead imagine it's an array of authors instead. Then their selector would give you a list of all their favourite drinks, which would require a loop in normal JS.
I think I'd prefer something more XPath-like than CSS myself (as someone linked in another comment)
Internally I'd imagine the language will have to loop through the json anyway. Are there any other benefits I'm missing (apart from of course the nice css-like syntax, which is perfectly valid).
It could be, yes. `map` isn't built-in on all browsers, though, which I think has hurt its use. i.e., most people don't think about it, so it doesn't get used as often.
Underscore.js and Prototype.js both define this function. You can include this and use map. Underscore delegates it to ECMAScript 5's native map if available.
It's definitely 'out there', and you can always just copy-paste to get the same code and delegation[1], but the lack of default availability is a pretty damaging blow. It shouldn't be, but the Javascript world isn't exactly a pantheon of high-quality code.