1) We are obviously aware of XMPP, having been developing client-side interfaces to it for years.
2) We started this project in 2001 - around the same time Jabber was in its infancy - and as smacktoward notes, the main reason we're not using XMPP today is inertia. XMPP is great, but XML just isn't our personal preference.
We figure being more open with our protocol can't be a bad thing and so it's out there for folks to pick apart and send us feedback, which we'd love! Alot of our protocol was built with an eye towards XMPP federation as well; I actually had Trillian interoperating with Google Talk one afternoon but it seems like that ship may have sailed. :-\