[Too]

    function getItem(id) {
      return $.get("/api/item/" + id);
    }
    $.when(getItem(3), getItem(4)).then(handleSuccess, handleFailure);

Maybe I'm taking the point of the example out of context here, but i worry that promises makes it too easy to write non thought through code like this. Why make two separate http-requests for something that should have been possible to do with one?

Future prediction: Someone will create code similar to the one above but inside a for-loop calling getItem hundreds of times instead of just 2.

[ajanuary]

You frequently don't control the API's your hitting. I've seen plenty of API's that don't support bulk operations.

I'm not sure I'm convinced if someone doesn't know how to do a bulk operation they're more likely to look because they've got an extra callback or two.