While I find this interesting, the idea is hardly new: separate the content (usually generated by scripts the programmer writes) from the layout/markup (generated by an "artist" or designer). Much of this today can be achieved with or without XML/XSL by using (X)HTML + CSS with a good backend process for generating content (perl, of course!). Good programmer/designers do this, and good production houses already separate the jobs.
I also feel that a good manager will develop a decent specification that will tell (1) the artist/designer what needs to be on the various pages of a site, including types of content and general layout guidelines, and (2) the programmer what sorts of data need to be consumed and the general output that should be thrown to the layout. If this is done correctly, your "XML as Wall of Separation" argument shouldn't need to be raised at all.
I *do* like your table regarding "Who Do What"; it clearly and cleanly shows that architectural changes to a site affect *everybody* and not just the programmer OR artist. This is too often forgotten.