I've been wondering that myself. I got a hint from an article in VBPJ by Deb Kurtura (sp?). Although she didn't go into detail, she suggested that XML provides one more mechanism for transferring data between tiers. For example, if the data access tier needs to pass all kinds of data to the business object tier, the choices include setting properties (usually very bad), using UDT's, or sending a string variable and then parsing out the individual data elements. XML provides for creating a standardized structure that is can then be passed as a single string between tiers.
I don't have the experience to know whether this represents a desirable direction in which to go or not.