I don't think XML is any better than HTML for presentation. In fact, HAML is way better, and much more powerful. In the same way I don't see why to use HSS when we have SASS/SCSS, Jade, Less...
I believe that point of AXR is to replace this underlying base, with something more suitable for web applications. You could still use a metalanguage on top of it (maybe with some tweaking) - you can write XML using HAML anyway.
Content is best represented with some mixture of properly-chosen DSLs. For example, Markdown-like language for texts, CSV-like language for tabular data, and so on.
There are too many various type of "content", so if you try to use lowest common denominator (like XML is) you: 1) lack the expression and have to type/generate lot of pointless code and 2) start mixing various types of content in a wrong ways (like tables for layouts, padding with spaces, CSS sprites etc).