I know this is cynical but might a degraded experience for your Android contacts be a significant plus of this feature? (edit: I mean for Apple Inc, not for the end user). Like green bubbles.
Maybe, to the extent that people care enough about it to bug their android-owning contacts about it. I don't think that will be a very large number of users, especially since this is pretty trivial compared to the features you lose in iMessage group chats by adding SMS contacts.