Personally, I find myself conflicted on this one.
In principle, I'm all for open cross-platform messaging. If it were up to me, I'd love to go back to the days of Trillian where you could interface with all of the major chat networks using a single client. As much of a pain as multiple networks is, it stops mattering when everything is accessible in the same client. Beeper Mini doesn't go that far, of course, but it would be one step closer. And as an added bonus, this is easy E2EE, which is always great to have in a network.
On the other hand, apparently the program was relying on an Apple code module ripped from an older version of macOS, which is how it was able to register with Apple's servers to begin with. So this wasn't a clean room reverse engineered implementation of the complete iMessage stack. Redistributing OS components in end-user software just kind of scummy - you didn't earn the knowledge for yourself - and goes against the traditional hacker ethos of making a product based on someone else's spec (e.g. BIOS).
On the third hand, I'm not sure I want any random yahoo having access to the iMessage network. iMessage spam basically doesn't exist; it's quite nice. I'm not sure Apple is prepared for, or ever could fully handle, the spam that would come with an open network.