I agree that GWT is not friendly to UI people who are used to writing their own markup. But I would argue that a good UX person should be concerned with how the user interacts with the application (not necessarily by writing HTML and CSS by hand, but by sketching out the design on paper or Illustrator), and a framework like GWT often makes it simple to build complex UIs that would be difficult/labor-intensive to create and maintain with a traditional web dev stack. A decent developer should be capable of taking mockups from a designer and building out the rounded corners and other pretty bits himself in CSS.