Talked with @wycats in IRC about this one, basically, here is the scenario:

Say the canonical blog example has two models, a Post and an Author. The Post model needs an Author upon creation, otherwise the response is rejected. This means we cannot do a PATCH request later to add the relationships. For reasons unknown, the Post will not assume a default Author, or use the currently authenticated user as the Author. How do we create a relationship when POSTing to the server to create the new document?

Perhaps @wycats can expound upon what he meant by "sideposting" here.