Major We have encountered several situations where we know at design time that an entity will have an association to another entity (in the same or a different service), but we don't know the exact type of the related entity until run-time.
One example are notifications about "events" for some entity the consumer is interested in. The "events" service will be stable over time and may notify about events on business entities that weren't even modeled at the time the "events" service was published.
This is comparable to defining Named Resource Properties with the additional information that the named resource is guaranteed to be an OData entity or a collection of OData entities.