If you use XML to hold the data, it's easier for people to use the data, as opposed to digging it out of HTML. XSLT will handle the presentation of the data, without any need to alter the underlying format of the data.
If I understand how it works correctly, content is provided as XML. If the consumer is a browser, it uses the style sheet in presenting the data. Other consumers can work with the data without being burdened with any stylistic fluff getting between them and the data.
ASP.NET can spew any form of data (that can be represented in binary, of course) back to the requester: XML; HTML; plaintext; grapics; whatever... It doesn't matter to it what you want to send.
If the pages are data centric, then it makes very good sense to use XML, as it makes the data more accessible. If, on the other hand, the pages contain an assortment of stuff that is unlikely to be processed by anything other than the human mind, then the case for XML isn't as compelling.
The two primary tasks you'd face would be: retrieving the data as XML and creating the style sheet to present the data as desired. I don't know how much or little a PITA developing the style sheets would present, but generating the XML will (eventually, at least) be a piece of cake.