The world is becoming an orgy of self-sacrifice when it is precisely the self that should not be sacrificed.
I'm not sure I understand your post so let me double check to see that I've got it. Are you asking why we might use XML within Ajax instead of plain text?
If that's the question, my response would be that many applications already return XML content so Ajax can harness that directly. For example, Web services return content using XML vocabularies so we can proxy the data server-side and consume it with Ajax. We can also return some database calls directly in XML content rather than plain text.
XML also gives us a much more structured approach to data than plain text. We can include information about hierarchies within the data and use tags to describe the content. It's often quite difficult to describe array data using plain text.
A final point is that XML allows us to take advantage of predefined vocabularies. These might be specific to an industry and allow Web and desktop applications to consume the same content.
I hope that helps and that I've correctly interpreted your question.