Basically, both JSon and XML are used to store structured data. If your AJAX call only gets a single value (like "number of votes"), you could just fetch it and not use JSON or XML at all. But as soon as you need to fetch several values (like "positive votes" and "negative votes"), you need some structure.
Both JSON and XML support nesting data (like a.b.c=X) . JSON has the advantage of being lightweight and very very easy to parse in JS (as, basically, JSON is JS), which is why it is generally preferred over XML for AJAX applications.
Even in the very simple "only one value" case, I'd recommend directly using JSON as it provides room for easy extensibility, whereas a single value would not.
