Json is faster to parse and is smaller in size. Furthermore, Json is a bit easier for me to serialize/deserialize than XML. Furthermore, I have more experience working with Json than XML so I could write up the actual API much faster.
So, I guess it's a preference kind of thing, but also faster.
I was really just interested if lackattack had gotten it done. He mentioned above that it was on his todo list. That was about a year ago, I'm just wondering if anything was done or not. Judging by your response I'd say nothing was, well... XML it is I guess.
With the new management now, it would be great to see the API updated and things like JSON added. I imagine there are more than a few programmers such as myself keeping an eye on this and looking forward to developing a few new tools.