The idea of a HACKING file is 'stolen' from the Subversion project, as it's a particularly nice thing to have around. It will be the mother of many more specific topics.
Again, some of these issues have been resolved, but Xmpp4Js is mature enough that there is no reason to switch back now.
Xmpp4Js is built in such a way that there is room to experiment with that technology. It should be as simple as implementing another Xmpp4Js.Transport. Currently there is no major advantage to switching from BOSH, but scalability concerns in the future may necessitate exploring other avenues including Bayeaux.