This makes sense if the system content is document-centric. For much business data, however, this is problematic. Obstacles include the great volumes of existing non-XML data and the nature of the data stores typically used. The ability to store a document as such is not the same as the ability to find and access the data within a document in a useful and efficient way. XML within the database is still a seething area of debate.
Middle tiers can emit XML to clients.
Feeding XML data into an XSL processor provides presentation control. Different XSL stylesheets can present the same XML in different ways or formats. The same data could be shown in text or in a table, or it could be filtered or annotated. Presentation could be done in HTML, RTF, word processor format and so on. Combining XML and XSL leads to great flexibility in presentation form and media.