Yes it's optional, and yes you can use jQuery.ajax or whatever, but in reality, it's not quite that simple.

The official guides in Ember default to assuming you're using Ember-Data (though they do at least mention it explicitly). This despite Ember-Data being far from production ready (as stated on the Github page) and far too slow for dealing with large quantities of data. At the same time, the conventions that are found in Ember-Data appear to be built in to core Ember (it says as much in the guides).

As far as Ember-Data not counting against Ember - it may be larger in scope than other client side data libraries (or the M part of other MVC frameworks or whatever) but the fact is that they are most definitely comparable, as it's the way that Ember gets stuff off the server, and saves it back to the server. Without Ember-Data, Ember has no built in way of doing that, which counts against it. With Ember-Data, well, it's not really ready yet, which also counts against it.