[jashkenas]

I'd lean towards never, unless you have a case with really special semantics.

For run-of-the-mill attribute massaging, I'd just define a new method on the model specifically for that purpose. eg.

    book.loadFromAmazon(amazonJSON)

[bct]

Hmmm, interesting. Here's a concrete example:

I've got an 'Organization' model and a 'Person' model. I want to use them interchangeably in some templates, so I need to be able to get their names in a consistent way.

An Organization just has a 'name', but a Person has a 'first_name' and a 'last_name'. I've been using a 'displayName' method to encapsulate the difference; would it make more sense to do something like:

    var Person = Backbone.Model.extend({
      initialize: function() {
        var name = this.get('first_name') + ' ' + this.get('last_name');
        this.set({ name: name }, { silent: true });
      }
    });

[crescentfresh]

.displayName() seems like a perfect encapsulation IMO.