Mac already has a pretty good XMPP client (Psi/Psi+). I do not consider iChat or Adium true XMPP clients, but rather legacy clients with XMPP supports hacked in (in case of iChat) or added on top of (Adium). The problem with Psi (which I have been using for the past years, recently switching to Psi+) is that it has a horrible UI as far as matching the Mac ecosystem habits goes. That’s why I couldn’t persuade any of my co-workers to switch to Psi (each time they curse at iChat/Adium for not supporting a particular XMPP feature).
I too believe that consolidating the interface across platforms should be secondary to consolidating it with each particular OS standards. Even in large and OS/platform-mixed teams, it’s usually more common that each single member works on a single platform (mostly), rather than on many other platforms. I do understand the convenience of leveraging the universal Mozilla framework, but that’s a developer’s convenience that shouldn’t impact the real user. And there is zero benefit of the platform in for the user.
As of this day, there is no single XMPP client for Mac that would be fully compliant with XMPP and provide a native GUI. Even Swift (which I had hopes for) falls pretty short. That’s where you could have a big success IMO (even with a paid product).