According to http://www.w3.org/TR/html4/struct/links.html#adef-href, "This attribute specifies the location of a Web resource, thus defining a link between the current element (the source anchor) and the destination anchor defined by this attribute."
If we are requesting the specific element is because we already know its url, then it would be optional to set this attribute. However, it is important to be consistent. If all the other responses of our API include "href", then it would be expected to include this attribute and the value of it could be the same url of the object we want to get.