How to parse a collection of objects in Backbone?
Based on the way your parse function works you're worried about mutability. Your code can be much shorter. Also, your use of _.clone is wasted since it's only a shallow copy. I've rewritten parse code and now it looks like this:

// response is array of object 
// [{id:1, prop: null },{id:2, prop: "bar" }]

// the output parsed_response can be 
//[{id:1, prop: null, isProp: true },
// {id:2, prop: "bar", isProp: false }]

parse: function(response)
{
    return _.map(response, function(obj) {
        obj = _.clone(obj);
        obj.isProp = obj.prop !== "" &amp;&amp; obj.prop !== null;
        return obj;
    });
}


If you want further explanation let me know :)

The only thing I'm concerned with is how your prop works? Can you just check for a falsey value here (i.e. !obj.prop) or do you really need to explicitly check for an empty string or null?

In term of re-usability, you're covered here. parse() doesn't reference this so it can more or less be called statically from anywhere in your code. But be careful where you use it from, if you start calling this all over the place, it would make more sense to refactor this parse() function to a more generic object.