isValid() returns undefined instead of performing validation on partial form

[miphe]

I forked the fiddle example by thedersen and changed isValid(true) to isValid(), so that I do not force validation on the entire model.

http://jsfiddle.net/miphe/udXL5/513/

As I understand it, validation should happen on all this.model.attributes that has been updated, and if there was no validation then undefined is returned. However, when filling in half the form and setting the model as such, validation should happen, though we still get undefined returned by isValid().

Is this intended? Did I misunderstand the docs?