I don't see why anyone would expect this closed-beta product to not get blacklisted by Apple as soon as it gains any mainstream traction.
The disadvantage of not offering an API for iMessage is that there is no easy way to block clients. Apps like Bluebubbles have to work by being installed on an actual Mac and scraping messages from iMessage and Beeper uses a similar architecture[0]. Presumably they have a data centre full of actual macs (or VMs).