Spoke with gnarf about this, our idea is to have the lastPromise available on the collection for animate, so:

$elements.animate( properties ).promise().then(
    function() { 
         /* gets called when ALL animations are complete */ 
    });

In this case, using with $.when would probably be the most advised route:

$.when( $elements.animate( properties ) ).then( fn );

The benefit would not only be for that particular animation set, but building a collection of them as well and being able to .then( fn ) once they are all done.