Jon Udell at InfoWorld - Styles of Web application intermediation:

"Consider a purchase order represented as an XML document and governed by a policy that requires schema validity. We can enforce that policy either on the wire or on the desktop. One way buys you application-independent consistency. The other way lets you tailor your users' interactive experiences. These are complementary strategies.

Here's a less familiar but equally compelling scenario: You're leasing a Web-based application, it lacks a feature you need, and the developer won't cough it up in a timely manner. Because the application's user interface is delivered through the Web as XML packets alongside the protocol and data packets, you can tweak it globally or locally. Same benefits, same synergy."