JSON is great. It's an extremely concise, machine parsable yet still human readable format. It's the best thing to happen to data interchange since XML, and it makes XML looks like the big piece of dog crap that it is. (Seriously, angle brackets? And redundant closing tags? Who thought that was human readable?)
However, some of you may be surprised to know, JSON is not the only format that can be returned in an Ajax request. Don't get me wrong, if I make a request to the Twitter API or some other webservice, I'm not going to be happy if it returns XML. That stuff is a pain. But there are other uses for Ajax that just consuming raw data. Did you know you can return HTML?
For example, search results since the beginning of time have been paginated. The nice thing to do nowadays is to make an Ajax request when the user requests the next page so we can avoid reloading the header, footer, sidebar and whatever else in your page isn't a search result. Now you almost certainly rendered the first page of results on the server, using whatever templating language it is you use. Something like:
([/sarcasm] for the facetity impaired; it's a terrible affliction.)
When you're creating any sort of Ajax endpoint, just ask yourself one question, "Is anyone other than a script on this site going to consume this?" The answer is probably no, so I'm going to ask you a question in return, "What is wrong with you? Why would you want to generate HTML on the client by concatenating strings together instead of on the server with a nice templating language?"