Why doesn't NSManagedObject instances hold a strong reference to their NSManagedObjectContext?

It makes more sense for an NSManagedObjectContext to own its NSManagedObjects than the other way around.

Keep in mind that the context is like a draw pad with all its objects on it. If that context goes away, the objects are no longer valid. If the objects owned the context, then the context going away would do nothing to the objects and they still appeared to be valid. In other words: a context can exist without objects, objects can't exist without a context.

Of course, a hybrid model (where a context owns its objects and objects own their context) wouldn't work either, because then you would run into a retain cycle.

NSManagedObject instances are nearly useless without their context

They can be (though not necessarily), but remember that they do have a reference to their context! Presumably it's a weak reference, but a reference nonetheless. If that reference returns nil, the object is invalid. If you make sure your context stays around (which is what I did in my answer to the other question), you won't have any issues.

This is because you otherwise would get a retain cycle. The managed object context internally uses arrays and other containers which have references to the managed objects.

Possibly, this retain cycle cannot be "explicitly broken" that easily by the internal implementation of Core Data, so that this reference has to be weak.