These Beeper folk sound a bit entitled. As was repeatedly mentioned in the other thread [1], building production applications on top of an undocumented and unsupported (in terms of backwards compatability, etc) API is a nightmare that should be avoided. Apple has every right to change their API, if they do Beeper will go down, and Beeper will blame Apple. I understand Apple's incentives to not want to be in this situation.
I don't buy their "Beeper unequivocally makes things secure story" either. For one, I do not want to have my Apple ID login routed through a third party. I trust an established, trillion dollar company far more with that sensitive info than I do a fast-moving, eager-to-break-things startup. And the list goes on.
It's an impressive engineering effort, but I really don't believe they're entitled to parasite off the undocumented iMessage API.
[1]: https://news.ycombinator.com/item?id=38574888